Answer:
Frames of reference.
For measuring the physical quantities you have to rely on something as a reference point or a standard value. In the case of physical quantities that are vectors, and an abstract coordinate system is required to express it clearly.
Let us consider a body located on a ship at some point. We can measure its distance from a ship or from the terrace of the building. Hence, we are assigning a coordinate system with respect to a person standing on the ship and terrace. These coordinate systems are called the frames of reference.
While measuring the distance, both coordinate system gives specific values for the same location with regard to the frames of reference.
When the ship is moving, the person on the terrace observes that his values of the distance change. But the person on the ship did not observe any change with respect to his coordinate system.
Similarly, If a person considers a moving train as his frame of reference where his coordinate system is in motion, he finds the things that are stationary on earth seem to move.
The train is his frame of reference which has a velocity with respect to Earth.
Therefore, anything that is stationary with respect to the ground seems to be in motion with respect to his frame of reference.
