The difference between a hologram and red dot reflex sight is this...

In a red dot sight, the "dot" looks like it is right there on the glass. The "dot" of a hologram looks like it is floating out in space in front of your weapon. So, lets say you sight in your weapon at 50 yards and your holographic dot appears to be floating 50 yards in front of your gun, you are deadly accurate at 50 yards without making sight adjustments.

If the target looks like it is farther away from you than the floating dot, aim higher unless you have time to make adjustments. It is an easy way to judge distance when firing on the fly.

Hope that was written clear enough for you.