>>12422435They're bumper plates. The big thick ones like that are 45, but they have other lighter ones with the same diameter so they look heavier than they actually are.
It's to keep the bar at the same hight no matter the weight, and also they are safe to drop since they're rubber but according to /fit/ they are just used by crossfit fags to make people think they are using more weight than they are...never mind that they're used in the Olympics.