One way to move things is to slide them along another surface. Some surfaces make sliding easy, like sliding skis over snow, but some surfaces are hard to slide on, like trying to roller-skate over a cracked and bumpy sidewalk. The more bumpy or uneven a surface, the more movement is slowed down. This slowing force is known as friction. Wheels were one of the first mechanical inventions.
Gears are wheels with teeth.
They work together to make things move more quickly or slowly. Different sized gears work together. For example, a large gear wheel turning once can make a smaller wheel turn many times. The smaller gear can be used to make something turn faster but with less power. A larger gear can be used to make something turn slower with more strength.