Air resistance is a force that slows down an object as it moves through the air and can be seen as a force that works in the opposite direction to the forces that cause an object to move. So, for example, in the picture of a parachutist the force of gravity is pulling the person towards the Earth whilst the parachute is resisiting this downwards movement, slowing down movement.
Take a look at the video below to see what happens to falling objects when air restistance is removed. Most people think that heavier object fall faster than lighter object, for example a heavy ball and a feather. Without air resistance both fall at the same speed.