Its All Relative
How fast are you moving? Because you've stopped to read these words, you're not moving at all, right?
Earth is traveling at 107,000 kilometers (67,000 miles) an hour around the Sun. You are standing still, but only in relation to Earth. Relative to the Sun, you are traveling through space very quickly. Physicists use the phrase "relative motion" to convey the idea that whether an object is at rest or in motion depends on your point of view--or your "frame of reference."
Multiple Frames of Reference
A "frame of reference" sounds complicated, but it's really just a physicist's way of describing someone's point of view. Different frames of reference are defined in part by speed. Someone in a parked car is in a different frame of reference than someone in a car traveling at a constant speed. The person in the moving car is in a different frame of reference than a person going even faster in an airplane.
Einstein was not the first to conceive of relative motion. The idea of "relativity" can be traced back to Galileo Galilei, the 17th-century Italian astronomer. Einstein's Theory of Relativity expanded and refined Galileo's idea of relative motion.
Follow the Bouncing Ball
In this scenario, the person with the ball is moving relative to the observer. Although the person bounces the ball straight up and down, the observer sees the ball move in a zigzag path. Both are correct in their observations.
Now, both the person with the ball and the observer are moving, but because they are traveling with the same velocity, they are not moving relative to one another--they are in the same frame of reference. Both see the ball going straight up and down.