The Yellow Ribbon program allows universities to sign up to accept a certain number of GI Bill students. For anything that school charges over and above the top rate offered by the GI Bill itself, though, the school agrees to kick in a certain amount of funds to help cover it. If they do that, the government will match that amount, effectively doubling it.
And that’s enough, in many cases, to keep vets from having to pay out-of-pocket to attend those schools.