Friction is a force that occurs when two surfaces rub against each other. It can slow down or stop the movement of objects, like when you slide a book across a table. The rougher the surfaces are, the more friction there is. This is why it's harder to push a heavy box on a carpet than on a smooth floor. There are different types of friction, such as static friction, which keeps objects at rest, and kinetic friction, which acts on moving objects. For example, when you walk, your shoes create friction with the ground, helping you to move without slipping.