This is Newton's first law of motion. It means that an object that is moving will keep moving in the same direction at the same rate if something doesn't stop it. Imagine a ball rolling down a hill. If the ball doesn't hit something (a tree, for example) the ball will keep on rolling. This is in theory. In practice, the ball will be slowed down by friction. Imagine a ball being thrown in the air. In theory, the ball will keep flying in the same direction that it was thrown in unless it hits something. In practice, it will eventually fall because of gravity. This is a complicated answer, so please feel free to ask for clarification.