personal rank adjustment is how you compare to the people you played with. Meaning how your hidden mmr (different from your rank) is.

Let's say you're a diamond 1 player and every season you end up diamond 1, but then one season you have 10 really bad placements and end platinum. You will get personal rank adjustment +10 every game because it knows you should be higher. But let's say it assumes you should be lower you will get less.

Sometimes it does the opposite when you have completed a ton of games the season. Say you have over 100 games the season it will start giving personal rank adjustment based on you vs the players in your game. If you're the highest mmr (meaning you are banning) it will give you -2 because being the highest mmr you should be good enough to find a way to win the game. This one is pretty rare. As it's more common for people to be slightly outside their hidden mmr.