Grappling exchanges lose more viewers than anything else in the sport, mostly because the hierarchy is invisible if you do not know it. Once you can name the positions, ground rounds stop being a stalemate and start being a sequence with a direction.
Guard
The bottom fighter has their legs between themselves and the opponent. Closed guard has the ankles locked behind the opponent's back; open guard does not. Guard is a defensive position that carries real offence — submissions from the bottom are available, and a bottom fighter who is threatening is not simply losing.
Judges nonetheless tend to favour the top fighter, and are explicitly permitted to, which is why fighters work so hard to get up rather than settle for an active guard.
Half guard
The bottom fighter has one leg trapped. It is a transitional position more than a resting one: the top fighter is trying to free the leg and pass, the bottom fighter is trying to recover full guard or get back to their feet. Rounds are frequently decided by who wins the majority of these exchanges.
Side control
The top fighter is across the opponent's torso with the legs cleared. This is where the pass has succeeded. Striking options open up, the bottom fighter's ability to create distance is reduced, and the route to mount or the back is short. Judges score sustained side control well, and correctly — escaping it costs energy that does not come back.
Mount
The top fighter is astride the opponent's torso. It is the strongest striking position in the sport and one of the strongest submission positions. Time in mount is punishing to score against and punishing to sit under. A fighter who is mounted and surviving is not neutralising the round; they are losing it slowly.
The back
Taking the back with both hooks in is the most dominant position available, because the fighter underneath cannot see what is coming and has almost no offence. The rear-naked choke is the highest-percentage submission in the sport for exactly this reason. A round containing two minutes of back control is very difficult for the other fighter to win, whatever they landed standing.
Why the hierarchy matters to scoring
Judges are asked to weigh effective grappling, and effectiveness here is largely about advancement and threat. A fighter who takes someone down and holds half guard for three minutes has done far less than a fighter who took them down, passed to mount, and threatened a finish — even though both spent the round "on top". Watching for advancement rather than for time on the mat is the single change that makes ground rounds legible.