It's the number of goals a team would expect to score given the chances they created in a game. Easy chances have a high xg. 1 would be a certain goal and should never happen 0 is no chance of scoring and, probably shouldn't happen either,unless it's an own goal, I suppose with no opposition player involved.
If a team has 3 chances all with an xg of 0.5 it means 50% of the time you would expect a player to score that chance. The teams xg for the game would be 1.5,even if they scored 0 or 4 goals.