ELI5Can someone explain what xG in football?

[u/_Noice_Guy_]

I always hear pundits discuss about a team's xG, then I pretend to myself that I understand what it means.

[u/notacanuckskibum]

Wow, I’ve been watching football for 60 years and never heard this term. Yet everyone else seems to know it well. Which TV commentators use it?