For those that don't really get it, it's used to determine how well a club or player is doing based on their performance in that season. xG is calculated based on how likely a team or player is to score over 90 minutes (or a season once it begins) based on their contributions to goals during games.
For example, Haaland at the moment has an xG of 1.1, meaning if he maintains his form and nothing changes, he'll score a goal every game, with the extra 0.1 adding an extra goal every ten games. So carrying on in this way, he's most likely going to end up on 41 goals. He could get a season ending injury, in which case it would drop to zero. Usually very good players hover around 0.8-1.0. If you think of it as percentages, it tells you the chances of them scoring.
Our highest xG player is Oscar (shocker) with 0.7. So if he plays 90 minutes, there's a 70% chance he'll score.
At club level, it's easier, xG gives you an idea of the attacking threat a team poses. Norwich sit top of the xG rankings for the Championship with 2.02 goals per game.
It'll come as no surprise that we're ranked 23rd in the league with 0.97, with Reading propping up the table and Huddersfield just above us. We're the worst ranked for goals against, with 1.78 per game. In fact we're worst ranked for nearly everything at the moment.