What? What do you mean “you know that X is the forward…”
Actor forward is the direction which your actor is facing. When you spawn it into the level, at 0 rotation, it may be 1 on X, 0 on Y. Then you rotate it by 90 degrees, and now the forward is 0 on X, 1 on Y.
Also, forward is a unit vector, meaning it’s length is always 1. Your example is wrong, forward cannot be 5 on X, 5 on Y.
For example, if you want to put an object 5 meters in front of your character, you grab the actor forward, multiply it with 500, add it to your actor’s location, and there you have it. This is how you use forward, and right.