The bridge, sometimes also called a 2-bridge, is an interior template with two stones. It is the equivalent of a diagonal connection on a Go board.
The idea is that when the opponent intrudes on the bridge by moving in one of the empty hexes between the pieces, you simply move in the other. Bridges are often very strong, and a quick way to advance across the board. They can sometimes be exploited, by Blue gaining a "free" move on one of the connecting hexes (as Red will probably want to respond by playing immediately on the other connecting hex, to secure her connection).