An object can be in motion relative to one set of object while at rest relative to some other set of object. Thus, rest and motion are relative
Example: Suppose you are sitting in a room. You are at rest in relation to all other stationary objects inside the room.But the room (or home) is on earth and the earth itself is not at rest. The earth revolves around the sun. It takes one year to complete one revolution around the sun. Thus, you are also revolving with the earth around the sun. Hence in relation to the sun you are in motion.