A good & simple explanation of
Expected Goals (xG) from BBC Sport, as used for the MOTD/MOTD2 analysis from this season onwards.
"Expected Goals tells us whether a player really should have scored"
"As well as helping us to assess player performance, Expected Goals also enables us to judge teams - and to predict future performance."
"Results were poor, but Expected Goal difference told a different story..."
"So if your team is performing above or below expectations, a look at their Expected Goals difference could tell you whether that run is likely to last."
Well worth a read if you're still unclear on what they mean:
http://www.bbc.co.uk/sport/football/40699431