Basically it is the ability to utilize your iron sights with your optic installed by simply aligning the rear sight to the front sight through the optic.
To help you avoid a newbie red dot issue that I had, I spent a lot of time trying to get my red dot to sit exactly on top of my front sight post. Accuracy, speed both suffered. When your red dot is on, just look at the dot. If it fails, then pop up your BUIS and just look at those.