Mount is a top position where you straddle your opponent's torso with both knees on the mat. Their hips are flat, their guard is gone, and they cannot use their legs offensively. It is worth four points in IBJJF competition — the highest positional score — and creates direct threats to the neck and arms.
From mount you can attack with armbar, triangle, ezekiel choke, and collar chokes. The bottom player must protect the neck, look to bridge-and-roll, and use the elbow-knee escape to recover half guard or closed guard.
Mount pressure is one of the hardest concepts for beginners to develop: staying mounted requires constant weight adjustment and anticipation of escapes.
For detailed coverage, see Mount.