Friction is a force that happens when two surfaces rub or slide against each other. It acts against movement and slows things down. Rough surfaces usually make more friction than smooth surfaces.
Friction can be helpful: it lets our shoes grip the ground and helps bikes stop when we use the brakes. It can also be a nuisance: it slows down things we want to keep moving.
Gravity is a force that pulls objects towards the Earth. It is why a dropped ball falls to the ground and why things have weight. Gravity pulls on everything, big or small.
Air resistance is a kind of friction from the air. It slows down things moving through the air, like a falling leaf or a parachute.