Closed guard is the entry point for most BJJ guard games. You lie on your back, wrap both legs around your opponent's waist, and lock your ankles behind their back. The locked position prevents them from stepping back to pass and puts you close enough to break their posture and attack.
From closed guard you can sweep, submit, or transition to open guard depending on what your opponent gives you. It is the position where most beginners first learn offensive grappling — hip escape, frame, and posture all become meaningful here.
For full coverage of attacking concepts and technique details, see Closed Guard.