Friction is a force that opposes motion. It is present whenever two surfaces rub over each other, such as when you rub your hands together, or when you apply the brakes on a bike or in a car. Friction also prevents an object from starting to move, such as a shoe placed on a ramp. When friction acts between two surfaces that are moving over each other, some kinetic energy is transformed into heat energy.
Friction can sometimes be useful. For example, we rely on the soles of our shoes to not slip over the ground that we are walking on, and the friction between a car tyre and the road surface helps cars to speed up, slow down and turn corners. Sometimes though, friction can be a nuisance. For example, the friction between a wheel and the axle that it rotates on wastes energy, so we try to minimise the friction using bearings and lubricants.