All measurements are relative to some standard.
Motion is measured relative to a reference frame. It is common to assume that this reference frame is at rest, or moving at constant velocity with respect to the fixed stars. Newton's laws of motion are defined with respect to a fixed reference frame. Everything in the room around you is in motion, anchored to the ground by gravity and continually spinning with the earth. There is no problem in assuming a fixed reference frame when measuring the movement of a ball during a game of catch, or how water drains from a sink.