Friction is resistance to sliding. Any two objects in contact with and trying to move relative to each other have friction. This can be low or high depending on the types of surface in contact. If two surfaces in contact are sliding, the friction produces heat. In braking systems friction is used to produce heat.
The process of creating this heat stops the motorcycle. The amount of friction between two surfaces depends on the materials and their roughness.
The amount of friction is described by a number called the coefficient of friction. This number is obtained by Friction Force being divided by the Perpendicular Force. To make matters more complicated the coefficient of friction has two different values. It is higher when there is no sliding but as soon as the surfaces start to move relative to each other the friction coefficient drops to a lower number.
This is why it is harder to start something sliding than it is to keep it sliding. After sliding starts it is called dynamic coefficient of friction. The higher the number the higher the amount of friction and the lower the number the lower the amount of friction.