There are four potential theories of distribution justice.
The feudal or caste system is the most unjust. Whether a person ends up being a prince or a slave depends solely on the luck at birth but not what he does.
The libertarian system or free market appears to offer equal opportunities, but this is not entirely so. If children from rich families are more likely to receive good education and job opportunities, they still have a huge advantage over poorer people.
In some countries, this potential flaw is tackled by the meritocratic approach. This attempts to offer fair equality of opportunity. For example, the government may provide equal educational opportunities for both the rich and the poor. In USA and China, children of ethnic minorities are even preferentially admitted to the college. (Of course, ethnic minorities are not necessarily poorer. That would be another topic.)
However, John Rawls (1921-2002) argued that even the meritocratic approach cannot totally eliminate inequality. You may provide opportunities to the disadvantaged, but you cannot prevent the fastest runners from winning a race and the cleverest students from entering college. To a certain degree, being fast or clever is also a matter of luck similar to being born in a rich family. The society still produces the retired professor who found no problem in charging his patient an outrageous sum.
Now, you may argue that your success is not the sole result of talent and upbringing; you have worked very hard to achieve what you have today. Rawls would counter by saying that even effort may be the product of a favorable upbringing. Besides, the society pays for your achievements, not your hard work. In this world, there must be soccer players who practise even harder than David Beckham and remain unheard of, but you probably would not advocate that they should be the richest soccer players instead.
Rawls is not saying that successful people should not earn more. This would take away the incentives to work hard and excel. Instead, he wants us to understand that while we are entitled to what we have, we should remain humble and not to think we deserve everything. Instead, earning more is justifiable only if we at the same time help less fortunate people.