Come on now, it is a valid question.
It's kind of hard to say what it is in terms of the game world, but I doubt it would be something that slows down the perception of time, because that would probably be a highly coveted weapons technology. I would just take a guess that it's a system for visually highlighting critical areas on a target's body and projecting the likelyhood of a hit given statistics the Pipboy records of it's user's combat abilities... But I'm probably reading way too far into things.
An easier answer, and the more likely one is the idea that in the Fallout world is that it's not science, it's
Science!