On applying brakes to a car it is moving down a hill, at a constant speed. In this process of motion with constant speed, brakes become hot. Why?

[prasanna]

If brakes were not applied, the car would have come down with acceleration. Hence, application of brakes is preventing this acceleration, i.e., brakes are putting frictional forces on the wheels. The work being done by these frictional forces shows up as heat.