Simple Definition:

Friction is the force that resists the movement of two objects that are in contact with each other. Friction occurs because the surfaces of objects are rough and the irregularities of the surfaces create resistance to movement. For example, when you try to slide a book across a table, the rough surface of the table creates friction that resists the book’s movement. The amount of friction depends on the roughness of the surfaces and the amount of force pushing the objects together.

Very Simple Definition:

The force that happens when two surfaces rub against each other, which slows them down or stops them from moving. For example, when you slide a pencil across a table, friction helps to keep the pencil in place and stops it from moving too fast.