The brake of your inline skates is an outgrowth made of rubber, hard plastic or polyurethane. It is fixed at the rear of the frame. Here is the story of this very useful accessory.
Whether in quad (traditional skates) or inline skating, the problematic of braking quickly emerged amongst skaters and inventors. A solution was found in the 18th century, using an outgrowth that rubs the ground to slow down the skater.