The operation of the brakes is basically as follows:
When we step on the brake pedal, a hydraulic pump is activated that drives the brake fluid to the brake callipers. The pressure of the liquid causes the pistons to push the pads. These pads make friction against the brake disc, where the kinetic energy is converted into dissipated heat, and thus the vehicle assembly gradually decelerates.
From here we can intuit some problems that we can find with the brakes: that the pads wear out is somewhat obvious, but also that the amount of heat generated by friction is too much to be dissipated correctly. These two situations cause deficiencies in braking.