The advanced stats on FBref.com and Stathead FBref are provided by Opta, and are based on their Expected Goals (xG) model. In addition to xG, other related "expected" stats that we have on our sites are Post-Shot xG (PSxG), xA (expected assists) and xAG (expected assisted goals).
More information on what is xG and how it is used can be found in this comprehensive explainer: