Very simplified answer for a 5 year old:
xG stands for “expected goals.”
Imagine your team misses a really easy shot, and you’re like “we should have scored!” Then later on, your rival scores a really difficult long-distance shot and you’re like “they got lucky, we should be up 1-0 instead of them!”
xG is basically putting a number on what the score “should” be. Therefore, higher is better since you want more goals.
As an example, if your xG is 1.5, then you “deserve” or “expect” to have scored 1.5 goals, so you’re lucky if you have 2 or more, and unlucky if you scored 0 or 1 in that game.
I’ll leave any discussion of how the math works to others 🙂
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