The Warp Pipe is one of the main modes of transportation of the Mushroom Kingdom. Generally speaking, a Warp Pipe is a pipe that doesn't always appear to be connected to a second pipe physically, although traveling through it will transport the traveler to the other end. In many cases, Warp Pipes can be used to cross great distances (and even worlds) instantly, while some pipes can't even transport anything.
In Mario & Luigi: Bowser's Inside Story, Bowser inhaled several warp pipes. In this game, they are used as a link between Bowser's Body and the Mushroom Kingdom. They are the only way to get the Mario brothers out of Bowser's body, and they can be found in the Pipe Yard. Bowser cannot jump into these.