Here’s a way to think of it:

If I steal all of your money and invest it to grow over time then I’ll end up with even more money while you don’t benefit from the growth that should have been yours. Now we have children and pass on our wealth. You pass on less because it was stolen, and I pass on more because of what I stole. This multiplies over the generations and a disparity is maintained. My offspring will have better educations and better opportunities because of the wealth they had access to, and yours will have fewer opportunities because you don’t have the money to spend on them.

The goal of reparations is to attempt to correct some of this disparity. It tries to provide opportunities for people who don’t have it but would have if something in the past weren’t stolen.

For an example that’s easy to see: In the US, black people are less likely to know how to swim. This has nothing to do with them being black, but what opportunities they had access to. This is for many reasons. One part of it is that most places had community pools, but they had restrictions for people of color. When this was outlawed, they instead just closed the pools or added memberships that required payment.

People also built up wealth over time through property, but black people were prevented from getting loans to buy property except in redlined places. This prevented them from building up generational wealth like white people were allowed to do. (This is ignoring the whole slavery thing…) It causes ripples through time where their children have less opportunities, which then causes their children to have fewer, and so on.