A term is called Relative term when it is compared with another similar term.
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 train..to 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.