Brake discs are pretty straight-forward parts, they provide a large surface area for brake pads to clamp onto to slow the car down. The larger the brake disc the larger the stopping surface area, this is why cars like Porsches and Ferraris have huge brake discs filling the inside of the wheel.
Brake discs can suffer from various problems such as corrosion and warping, continual maintenance and checks should be carried out to ensure they're in top condition, it's one of the main parts of the car that can have a direct impact on yours and other people's safety.