A reference frame is a perspective or coordinate system used to measure and describe the position and motion of objects. It provides a set of criteria, such as location and time, from which observations can be made. For example, when observing a moving car, one might use a stationary reference frame, like a sidewalk, to determine the car's speed and direction. Different reference frames can lead to different observations. For instance, in the context of Einstein's theory of relativity, an observer on a train moving at high speed will perceive time and distance differently than someone standing still on a platform. This illustrates how motion is relative to the chosen reference frame.