It is a tough question with a relative answer depending who you are, where you grew up and where you currently are living. There will, something those humans who have chosen to get children have experienced, never be something that is truly fair in real life but that everyone have a strong feeling of what fair is.
Even if what is fair is a philosophical question with a relative answer, all of us still have an idea of what fair is. That idea of fairness is something that probably has been with us since we parted ways with the apes, the true home economicus, 400 000 years ago and learned how to cooperate.
To make cooperation work, and get close to their goal of their shared intentionality, whatever game that was, our ancestors not only cared for their partners but also believed in the equivalence of oneself with others, splitting the rewards of the potential success equally. Otherwise no one wanted cooperate with you and otherwise you died. A “we greater than me” emerged and with this second-personal morality, where we related to others as equal partners sharing a common goal, the rules of cooperation turned, when humans grew into bigger social groups, into collective norms of morality, how to treat other people, and a sense of fairness was passed through generation to our present we.
What that fair is, is something we carry with us before it is taught to us and even though fair today is as relative as it was back then when we roamed the savannah, we can through some thought experiments and examples ballpark it.
It was probably perceived as unfair if someone didn’t throw any spear but walked in to get all the good spoils in the same way that if someone doing all the hunting hard work wouldn’t get anything.
Or with another more present day example, it probably gives an air of unfairness if the fortunate rich of us, having surrounded ourselves with more complex technosphere that consumes more energy and materials and leaves more waste, destroying the planet more than the rest of us (Carbon emissions of richest 1 percent more than double the emissions of the poorest half of humanity | Oxfam International) didn't take a fairer share of the responsibility of fixing of our planet.
We can go through these experimental questions to concretize fairness in not only energy use and taking responsibility for fixing our planet, but find fairness in access to health (like vaccination), in access to education, in access to water, in access to dishwashers, in access to food, in access to leisure, in access to meaningful activity and in access to the realization of meaningful social relationships.
It is important that we understand that fairness is more than just a feeling. How we answer that question of what is fair determines how we realize our goal of getting all humans to prosper within limits. Unfairness, inequality or unjust distribution of resources is in itself detrimental to our well-being. Having less than others inflicts the real physical pain of feeling socially excluded and trying to keep up with the Joneses in what actually is a zero sum game that destroys the planet.
A fair distribution is as important for us reaching our goal to prosper within in limits as it was for our ancestors because why should anyone care to do something if the efforts and rewards are not shared fairly? This was understood when combating dictators during the world war and it is imperative to understand when trying to live within limits.
The limits of our planet also bring in fairness in a more tangible way. When our economic models had us to believe in endless expansion they obscured the limits and the delicate issue of sharing fair. But as everyone sharing a beautiful delicious cake has experienced one person cannot satisfy sweet teeth as much as they want without affecting their fellow cake eaters chances of tasting the cake. In the same way the resources of our beautiful earth are limited and therefore there is also a limit to the potential material comfort everyone can get.
Thus before we can distribute we first have to know how big our cake is, find out the limits of the planet, the limits of our resources, and establish ecological limits. Then the question of the quality versus quantity of life will have to be humanly dealt with, and innate in the question of fair distribution of resources to achieve a certain quality of life is that it encompasses the whole humanity and its fellow inhabitants on this planet.
As our current economy can be seen as anything but fair even though the economy and its fans have tried hardest to portray uneven transfer of resources, of exergy, from those without to those with, as fair. Instead of leveling the playing field the economy through the systems trap of "Success to the successful" has exacerbated the negative effect of inequality on our well-being.
So a first step to fixing fairness is fixing the economy because economy is about allocation resources. However the economy forgot scale and fairness and performing one of these thought experiments one would find it really hard to argue, under John Rawls' veil of ignorance, that fair is when one human being lucky to borne at a certain place with access to money or with the possibilities to use abilities to access money should get allocated more and more food or medicine while others get none.
To fix fairness we have to deal with both access to income and access to wealth from which incomes originates. We have to stop pampering the rich, treating them as delicate Ming vases. They are humans just as all of us, they have just been a bit more lucky, and should be treated as fairly as the rest of us.
Changing the economy to be fairer means facing a lot of norms regarding how society works. One these powerful ideas blocking a fairer society is the idea of private property. Its powerful influence over our lives is demonstrated in how the big ideological theories like Capitalism and Marxism evolved from discussing its existence and in how we humans have organized around this problematic mine and thine for a long time.
It is problematic because it perpetuate the systemic trap of "Success to the successful" and increases inequality. It enclosure to a few what could, without to much convincing, be considered common resources endowed to us all.
Luckily we human have also recognized the idea of sharing. Through public property like parks, libraries, schools and hospitals we understood that it is impractical in many ways that only one person owns all the trees, sea views (allemansrätten in Sweden Freedom to roam - Wikipedia), books, internet or hospitals. This impracticality of private property is now hindering us to flourish within limits with few get richer and a lot get poorer. Furthermore considering the limits of our planet it is impossible for 9 billion people to have their own SUVs.
There is no need to totally abolish owning but progress should be the progress of sharing. From bridges, rail roads, roads, schools we can and are learning to share SUVs, tools and so forth and this development can continue in many areas. The hardest part for those people accustomed to the norms of property will be sharing wealth like land and the ownership of companies.
But don’t be afraid the fortunate are not alone, and sharing is innate to us human beings, so to let us turn the theoretical thought experiments into practical pointers to find out what fair is together and adjust our economy and its ideas accordingly!