According to Newton's First Law of Motion, if an object is at rest, then it stays at rest as long as no external forces are applied to get the object moving. If an object is in motion, then it stays in motion (again as long as no external forces are applied to say otherwise). This is known as inertia.
For example, if you were to launch a baseball into deep space (far from any planet), then the ball will travel in a straight line at a constant speed until it either hits another object to have it slow down, or another planet's gravity starts to pull on the ball.
