PDO is basically a proxy for luck in hockey. The number comes from adding a player’s on-ice shooting percentage with their on-ice save percentage. For example, a player may look better than he actually is because his goalie is saving absolutely everything whenever he’s on the ice, with the chance it all comes crashing down rather abruptly. (This happened to the Flames’ third defensive pairing late in the season.)
xPDO, meanwhile is a player’s expected PDO. The general rule of thumb has been that a PDO of 100 means the player was neither lucky nor unlucky, and what should have happened happened; by adding the player’s expected Fenwick shooting percentage to his expected Fenwick save percentage, it gets narrowed down to an individual focus.