For example if we try to slide a wooden block across a table then friction acts in the opposite direction to the movement of the brick. The amount of friction will depend upon the nature of the two surfaces in contact with each other. The glass has a smooth surface and so slides easily across the polished bar top - there is little friction. This gives us an idea about how friction works. If we were rolling one object over another the friction still arises because of imperfections in the surfaces.
Friction then is a measure of the force pressing the two objects together.
Friction can be a very useful thing. The layer of ice and water reduces the normal level of friction we experience when we walk on the relatively rough surface of a path. The same applies to cars, countless accidents happen in icy weather when control on icy roads becomes difficult. On the other hand, imagine a motor which ran friction free. Without the external force of friction, once started the motor would run forever - perpetual motion.