A term is called Relative term when it is compared with another similar term.
take for example, 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 the passenger the train in at rest but he sees the platform speeding past him, meaning you appear to be in motion with respect to him...

this concept is very much relative, as the earth like all the other planets including the sun are actually moving. so though the computer in front of you may seem to be at rest with respect to you, it is actually in motion if someone from the moon or some other planet sees it.

We are comparing motions in each case. Thus we can say that motion is a relative term.