Newton’s first law of motion states that a body continues to be in a state of rest or of uniform motion in a straight line unless it is acted upon by a net external force.
The state of rest or motion of a body depends on its relative position with respect to an observer. A person in a running car is at rest with respect to another person in the same car. But the same person is in motion with respect to a person standing on the road. For this reason, it is necessary to record measurements of changes in position, velocity, acceleration and force with respect to a chosen frame of reference.
A reference frame relative to which a body in translatory motion has constant velocity, if no net external force acts on it, is known as an inertial frame of reference.