Rest and motion are called relative terms because both of them are defined with respect to each other. Without motion, there will be nothing at rest and vice-versa.
For example, assume you are in a closed vehicle moving at a uniform speed. Unless you see outside environments such as a tree or other objects you won't realize that you are moving.
The other example would be the earth. If we were not to look at the sun, stars, and the moon up in the sky.. we would never realize that we are in motion relative to the sun, moon, and the stars.