[He quickly mentally reminds himself to put this in less technical terms than usual.]

Power Armor is as it sounds, full-body combat armor that fully encases the user within a mechanical frame. Rather than strapping pieces of armor to the body, think of it as a mechanical "shell" that has armor plates affixed to the outside. The frame is powered with a fusion core, and utilizes hydraulics to help the user move and more easily carry the weight of the suit.

[Hey, that could have been much worse.]

I've been training and fighting in it for over a decade. It's the Brotherhood of Steel's iconic weapon of choice.