Imagine that a racist babysitter is watching after a white kid and a black kid. And she has two cookies that she's going to give to the kids. She likes the white kid better so she breaks one of the cookies in half and gives the white kid one and a half cookies while giving the black kid half a cookie. This isn't the white kids fault - they didn't ask for more cookie than the black kid, it just happened whether they wanted it or not. But if they don't give that extra half to the black kid then they are just as guilty as the racist babysitter of making the black kid's life worse.
It can be hard to give that extra piece back. After all it was just given to you, shouldn't you be able to keep what you were given? no, you shouldn't, because it wasn't just, it was wrong. Just because you didn't do the initial action of stealing the cookie, doesn't mean that it was okay for you to have one and a half cookies while the other kid gets only a half.
If that white kid doesn't want to benefit from racism, they have to not only give that extra half back, they have to tell their parents to fire the racist babysitter, and until they're fired, the kid has to keep giving it back every day and telling their parent every day. In this analogy, the white kid is white people of today, the black kid is the black people of today, the babysitter is the white people who started this shit, and the parents are the structure of institutional racism. If you as the white kid do nothing, you benefit from racism. This is why it is white people's responsibility to fix racism (which is way more complicated than the cookie situation).
It's white people's responsibility to undo racism.
It's men's responsibility to undo sexism.
It's cis people's responsibility to undo transphobia and cissexism.
It's non-disabled people's responsibility to undo ableism.
It's straight people's responsibility to undo homophobia and heterocentrism.
And so on.