A flag (or "flagging") is a body-positioning technique where the climber extends one leg outward — usually behind or across the body — to balance their weight when reaching with the opposite hand. The flagged leg has no weight on it; it's pure counterbalance against the rotational force created by reaching across the body.
Three main types: the inside flag (leg crosses inside, behind the standing leg), the outside flag (leg trails outside, often behind), and the back flag (leg trails behind in space). Climbers choose based on which side the next hand-hold is on and which side has a usable foot-hold.
Flagging is fundamental to efficient climbing. Without it, a climber reaching with the right hand from a left-foot hold would barn-door (rotate uncontrollably away from the wall). The flag stops the rotation, conserves arm strength, and lets the climber move smoothly rather than fighting the swing. Beginners learn flagging within their first sessions and never stop using it.