Newton's First Law: A body in the state of rest or uniform motion do not change its state unless acted upon by an unbalanced force.
It means if a body is at rest, then it will continue to be in that resting position until an unbalanced force acts on it.
And if a body is in uniform motion(i.e. having constant speed and a fixed direction), then it will continue to be in that uniform motion unless an unbalanced force acts on it.
So, in order to change the state of motion of the body, an unbalanced force must act on it.