Newton's 1st Law tells us that a body in motion will continue to move in a straight line at a constant speed unless acted on by a force. Yet when we roll or slide things along a surface they do slow down and stop, so some force must be acting to stop it. The "force" that slows down and stops the movement is friction. Friction is a force that resists motion along or through it (in the case of air or a fluid).

In the case of an object sliding along a surface, the friction is proportional to the "load" or weight of the object on the surface: F_{f} = μF_{n} where F_{f} is the force of friction, F_{n} is the load or gravitational force (weight) of the object on the surface, and μ is a constant that's different for every type of surface (glass, metal, etc.). Note that if the surface is slanted (inclined plane), F_{n} will be the force normal to the incline.

There are other types of friction (static, fluid, etc) but this is the basic idea. Hope that helps.