A war bridle is basically a rope or cord that runs through a horse's mouth, around the chin and over the poll.
They are one of the harshest devices that can be used on a horse. (They're made in a similar way to English rope halters, but those don't go through the mouth). In fact, many people think a war bridle is inherently abusive.
However, they are sometimes used by experienced horsemen, especially western, to control a "rogue" horse or one that is doing things which are dangerous, when no other tool is working.