Now that we have this System know that it is in motion and the objects within it are also in motion. With this in mind think of the motion or moreover the dynamics of the motion. I say dynamics due to the time dependency of relativity. This is the key to understanding. The theory of relativity help in understanding the complex idea of position with respect to Time of an object in a system with respect to other objects within that system.
All objects are to be looked at as the makings of a system.
This is the key to understanding.