Its a type of building that spawns in woods and fields far from roads and cities. It consists of a small entrance linking to a basement with some weapons/tools/food and a safe source of infinite water. All in all, their often secluded location often makes them impractical as a base as you’ll have to waste a lot of time each day to go from your base to looting grounds.
Best way to find them is to hack a console in a lab and reveal the nearby area. You often spot at least one every time you do that.