The relativity of simultaneity

The relativity of simultaneity isn't absolute, but depends on the observer's reference frame. 

Anyway, it is easiest to explain with an example:

Two observers, Tom and William. 

Tom is on a train car. The train goes too faster but always in constant speed. Tom is in the center of the train car and he throws a ray of light. That ray of light arrives at the walls in the same time. For Tom, they are simultaneous events. It is in the following image. 

William is on the train station. William sees two movements, the ray of light and the train. (I remember you that the speed of light is constant in all references systems).

William observe that the ray of light traveling in the inverse direction of the movement, reachs the wall before the ray of light which is traveling in the direction of the movement. We can see it better in the following image. 

In conclusion, we can analyze that all we see depends of the position of the observer. There is a different reality for each observer and the simultaneity is relative.