Answer:
Reference point and reference frame
To describe the position of an object we need a reference point or origin. An object may seem to be moving to one observer and stationary to another.
Example: A passenger inside a bus sees the other passengers to be at rest, whereas an observer outside the bus sees the passengers to be in motion.
In order to make observations easy, a convention or a common reference point or frame is needed. All objects must be in the same reference frame.