Two guards on the same side of the floor in transition. The point guard pushes the ball at the wing, and what follows is a handoff, a ball screen, or a fake of either. The trailing big arrives a beat later and gives the pair a third option behind the ball.
Why coaches use it
It is early offense you do not have to call. Two players who know the series can start it off a rebound, and by the time the defense has matched up, the action is already happening.
It is also a family rather than a single play, which is what makes it hold up over a season. The same start produces a handoff, a keep, a flare for the guard who gave it up, or a ball screen from the trailer. Defenses that have scouted one of those are still wrong about the other three.
The naming is not universal. Some programs number the variants — 21, 22 and so on, usually after the two guards involved — and others just call the whole thing pistol. Do not assume a coach from another gym means the same thing you do.
How it is guarded
The handoff is guarded like any handoff: over, under, or switch. The harder question is the trailing big. If his defender steps up to help on the exchange, the flare or the ball screen behind it is open, and the skip pass is a long closeout away.
Most teams push the ball toward the sideline early so the pistol has less room to work, then switch the guard-to-guard exchange to keep a body in front of the ball.