xG, or expected goals, is a statistic that rates the quality of a chance as a number between 0 and 1. It works by comparing a shot to thousands of past shots from similar positions and situations, then giving the probability that one like it ends in a goal.
The measure came out of football analytics in the 2000s and reached a mainstream audience when the BBC's Match of the Day started putting it on screen in 2017. A shot from the penalty spot is worth about 0.76 xG, because historically around three of every four penalties are scored.
Its value is in the long run. A side can lose a match while creating the better chances, but over a season a team's xG tends to track its results more closely than any single scoreline does, which is why analysts use it to judge whether a hot or cold run is likely to hold.
It has limits. xG says nothing about who took the shot or how cleanly they struck it, and a model is only as good as the data behind it. Read as a guide to chance quality rather than a verdict on a result, it is one of the clearest numbers in the modern game.