This statement is known as Newton's First Law . It took people from 350BC to about 1600AD in order to figure it out. Objects tend to resist changes in their motion. And such interactions are described by forces. Newton's Second Law Galileo and Newton realized that an object's velocity does not change unless its interactions with its environment result in a net force.
Forces cause acceleration.
All that matters is how strong is the net force and in what direction it acts. We know from experience that the answer depends on the object. Different objects resist changes in their motion differently. You have more trouble diverting a truck from its course as compared to a fly. We say that objects have inertia , a term used to describe their tendency to resist changes.