Friction happens when things move against each other. The tiny particles that things are made of, atoms and molecules, push on the particles in something moving across it. This makes them vibrate, or wiggle, and when particles wiggle we call this heat. When something heats up, it takes energy, and the energy has to come from somewhere. (Remember that energy is neither created nor destroyed.) In this case the energy is taken from the motion of the objects, and they have to slow down.
A lot of scientists have tried hard to get rid of friction in their equipment, but sometimes friction is very useful. If there was no friction, a car or bicycle would not be able to start up or slow down. The ground, floor, or whatever you are standing on would be slipperier than ice. Pencils would have to be shaped differently or they would just slide out of your fingers, but that wouldn't matter because without friction, a pencil wouldn't make a mark on paper anyway. It is more fun to think about than to experience, I bet.
