Relative Motion is a topic about something like this: Imaging you are standing still on the road and you see a bus moving at speed of 20 m/s with respect to you. A guy in the bus was walking from front to rear of the bus at 1m/s with respect to the moving bus. So what is the velocity of the guy walking in the bus with respect to you?

In this question if the guy is point P, you are point A, and the bus is point B, you get the following relationship:

                      V_PA=V_PB+V_BA

                     V_PA=(-1m/s) + (20)=19 m/s.

  V_PA means the velocity of P with respect to A, which is the velocity the guy with respect to you.

  V_PB means the velocity of P with respect to B, which is the velocity of the guy with respect to the bus.

  V_BA is then the velocity of B with respect to A, in other words, the velocity of the bus with respect the you.

The velocity of an object depends on the reference frame of whoever is observing or measuring the velocity. And that is why velocity with respect to different things are different, just like the example above, the guy in the bus thinks that he is moving at -1 m/s (with respect to the moving bus), while you, who stands still on the road, sees him moving at 19 m/s (with respect to you).