Friction doesn't only occur when two solid objects come into contact with each other. It also happens when an object moves through the air. You can't see air but that doesn't mean there is nothing there. It is made up of many very tiny particles and when an object moves through the air it bumps into these particles. This causes friction and it slows down the moving object.
Something that is made to travel through the air, such as an aircraft, is designed to have as little air resistance as possible.
This worksheet should help you test your knowledge of air resistance.