Newton's first law states, "Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it." A more colloquial way of saying it is "an object in motion stays in motion."
Newton's first law, one of his three laws of motion, describes inertia. For example, if a ball is set in motion, it keeps going unless a force stops it from going farther. A number of forces could stop it, such as friction as the ball rolls across the surface. Other things could make it go faster, such as a fan blowing behind it or gravity pulling it down a slope. Similarly, an object that is not moving stays still unless something acts upon it to move it, such as a cat's paw swiping at the ball. Newton describes how velocity changes in his second law.