When describing relative plate motion, several different reference frames are possible. The simpliest one, used by most science textbooks is the boundary reference frame. With this reference frame, the relative motion of plates is observed from the plate boundary.

 

When you are standing at the side of a road, you are in a fixed location. Cars move relative to you.

 

Imagine you are a fixed observer standing at the boundary between two plates. As indicated by the red arrows, plates might be moving towards you, away from you, or past you.