Newton's first law states that:
"A body at rest will remain at rest unless acted on by an external force"
"a body in motion will remain in motion unless acted on by an external force".
In astronomical terms, this means that in space bodies continue in straight lines unless a force external to the body acts on them, and stationary bodies do not spontaneously start to move. "In motion" in this context actually means at constant velocity, ie their speed and direction of their motion do not change with time.
External forces at the macroscopic level can be gravitational forces or mechanical forces due to collisions.