Self-propelled means able to move by its own power, without being pushed, pulled, or carried by something else.

A self-propelled lawn mower has its own engine that turns the wheels, so you just guide it instead of having to push it across the yard. A self-propelled artillery gun can drive itself to different positions on a battlefield, unlike older cannons that had to be towed by horses or trucks. Many vacuum cleaners are self-propelled, using small motors to help them glide forward so you don't have to drag them around.

The key is that the power comes from inside the machine itself. A wagon isn't self-propelled because someone has to pull it. A bicycle isn't self-propelled because you have to pedal it. But a car is self-propelled because its engine provides the force to move it forward.

Before self-propelled machines, almost everything was either pushed by people, pulled by animals, or moved by wind or water. Self-propelled machines changed the world by letting people move heavy loads quickly without relying on human or animal muscle power.