explain how a body can be simultaneously at rest and in motion.
A body can be simultaneously at rest and in motion as it depends on the reference point by which the body is being observed.
Motion and rest are relative concepts. There is no absolute rest. We can define the state of rest or motion only with respect to another object or a point in space taken as reference.
Example: Suppose you are in a train and the train is moving but you are sitting still in the train. Your copassenger will see you still i.e. in rest but someone from the station or outside of the train will see you moving i.e. in motion with the train.