It's a joining procedure with military origins. It was designed to expedite an aircraft's landing from cruise speed and to minimise the possibility of a wartime enemy aircraft from getting a clean shot at it.
Normally flown via "Initials"; a point about two miles out from the threshold, on the deadside of the runway. R/T "Initials, run and break"
Run in up the deadside, parallel to the runway. At the appropriate moment, roll into a steep level turn, to cross over the runway to the downwind leg. Throttle closed, airbrakes out if appropriate. Gear down / flaps as the speed drops. Normal landing. Do it without opening the throttle again if you're really good at judging it.
R/T "On the break" or similar, depending on local orders, followed by "Downwind" and "Finals" as normal.
Not a good thing to do if there are inexperienced pilots in the circuit.