The PIE formula compiles everything a single player does in a game -- points scored, rebounds, blocks, missed free throws etc. -- and weighs that number against the same stats generated by everyone in that same game. For the mathematically-minded, the individual player’s stats are the numerator (top) of this equation and the cumulative stats of everyone in the game are the denominator (bottom). The formula then computes a percent value for each player which gives us, in laymen’s terms, the percentage of positive things attributable to that player in that game.
http://masseybasketball.blogspot.com/2013/07/player-impact-estimate.html
NBA.com distills it down to a team rating but it's just measuring how efficient teams have been at a player level. For context, GSW had a PIE rating around 58 for the season last year. Through two games, the Cavs are #6 in the league at 56.3.