Bumper plate

In plain words

The rubber is there so a loaded bar can be dropped to the floor. It covers a plate of a fixed size: every bumper has the same diameter, whatever the number on it. The largest competition discs measure 450 mm across, within a millimetre, and must be covered in rubber or plastic, by the International Weightlifting Federation's rules. So a bar loaded with 10 kg bumpers sits at the same height as one loaded with 25 kg plates. Ordinary small iron plates would leave the bar lower, so a deadlift starts nearer the floor.

An example

The figure shows a 20 kg bar with one 25 kg bumper on each side: 70 kg. The federation's colour code runs 25 kg red, 20 kg blue, 15 kg yellow and 10 kg green, so those two discs are red. Swap them for green 10 kg bumpers and the bar weighs 40 kg, but it sits at the same height off the floor.