Issac Newton came up with three laws that explain motion. His first law of motion states “an object in motion says in motion and an object at rest stays at rest unless acted upon by an outside force.” This is also called the law of inertia. Inertia is the ability of an object to resist change in motion. The more mass an object has the more force it will take to get the object moving or stop the object.
A basketball is sitting on the gym floor (an object at rest). It will continue to just sit there until someone (an outside force) moves it. Lets say you kick the ball and it is now an object in motion. It will continue moving until acted upon by an outside force (hitting a wall, somebody else stopping it, friction between the ball and the floor, etc)