Motion is a relative term. When we say that a body is in motion, it basically changes its position with respect to other bodies around it or with respect to any fixed point known as the reference point. Let's understand this with an example.
Suppose, you are standing on a platform and see a train speeding past you.The train is definitely in motion with respect to you.
Now, think of a passenger in the same speeding train. For him, the train is at rest but he sees the platform speeding past him, meaning you appear to be in motion with respect to him.
This concept of motion is very much relative and depends on the observer.