Galileo discovered a very remarkable fact about motion... that is the principle of inertia -- if something is moving, with nothing touching it and completely undisturbed, it will go on forever, coasting at a uniform speed in a straight line.
Newton modify this idea, saying the only way to change the motion of a body used to use force. If the body speeds up, a force has been applied in the direction of the motion. On the other hand, if its motion is changed to a new direction, a force has been applied sideways. Newton thus added the idea that a force is needed to change the speed or the direction of motion of a body.