Brakes are the part of a bike that slows you down. They come in several varieties, and the best type in the rim brake category is linear pull brakes, also known as V-Brakes. They work by using two straight arms, one on each side of the wheels, that have a pivot attached to the bike frame on the bottom and a brake pad near the bottom. The two arms are connected by the brake cable, on one side by a clamp and the other side by an arm that fits around the end of the brake housing.
The power of linear pull brakes is determined by two factors, the stiffness of the brake arms, as if they flex they won't transmit braking force to the rim. The stiffness of the bike frame, because when the pad touches the rim the brake mounts are pushed outwards. This problem can be fixed with a brake booster, a metal arch which holds the pivots together. The type of material the pad and rim is made out of is also important.