A doorknob is called a “wheel and axle” because it has both a wheel (the knob) and an axle (the rod in the door) that turn on the same axis. In some situations, like a doorknob, a wheel and axle can also act as a lever.
People often think of a wheel and axle as a way to make it easier to roll something from one place to another by reducing friction. This is the type of wheel-and-axle system that is used in cars and bicycles. When the object is set up this way, a force is applied to it, and the object is easier to move because the wheel has a smaller surface area and is smaller than the axle. Most of the time, a bigger wheel makes it easier to roll something.
You can use the wheel and axle as a kind of lever. When you put force on the outside edge of a wheel, you get more torque on the axle than when you put the same force on an axle without a wheel. This is because the wheel works like a lever to give the machine an edge. The more mechanical advantage a wheel has, and the bigger it is, the easier it is to turn the axle. This is how a doorknob makes a wheel-and-axle system.