There is no such thing as simultaneity between two events when viewed in different frames of reference. If you understand what we have talked about so far, this concept will be a breeze. First let's clarify what this concept is stating. If Meagan sees two events happen at the same time for her frame of reference, Garret, who is moving with respect to Meagan, will not see the events occur at the same time. Let's use another example. Imagine that Meagan is standing outside and notices that there are two identical cannons 100 yards apart and facing each other. All of the sudden, both cannons fire at the same time and the cannonballs smash into each other at exactly half their distance, 50 yards. This is no surprise since, the cannons are identical and they fire cannonballs at the same speed. Now, suppose that Garret was riding his skateboard super fast towards one of the cannons, and he was directly in the line of fire for both. Also suppose he was exactly half way between the two cannons when they fired. What would happen? The cannonball that Garret was moving towards would hit him first. It had less distance to travel since he was moving towards it.
Now, let's replace the cannons with light bulbs that turn on at the same time in Meagan's frame of reference. If Garret rides his skateboard in the same fashion as he did with the cannonballs, when he reaches the halfway mark, he sees the light bulb he is moving towards turn on first and then he sees the light bulb he is moving away from turn on last. See Fig 6 below for clarification.
In Fig 6, the bulb on the right turns on first. I have shown Garret to be moving in the same direction of the distance line between the bulbs, and he is looking towards the moon. As stated earlier, when the bulbs turn on in Meagan's frame of reference, Garret will see the bulb on the right turn on before the bulb on the left does. Since he is moving toward the bulb on the right, its light has a shorter distance to travel to reach him. Garret would argue with Meagan that the bulbs did not turn on at the same time, but in Meagan's perspective they did. Hopefully, you can see how different frames of reference will not allow events to be observed as simultaneous.