It's a measure (here's last night's game) that explains, well, let's just rip off his explanation:
For example, if Player A skates a shift in which he takes a shot that misses the net, his teammate then puts one on goal and another teammate blocks a shot at the other end before the end of Player A's shift, Player A will have a +1 Corsi Rating for that shift. To put it in context, Henrik Zetterberg had the League's best Corsi Rating a season ago and a bunch of Thrashers were at the bottom. Basically it's another tool in the analytical arsenal to gauge a player's even strength effectiveness. Got it? Good
Now, as you've doubtlessly noticed, the Pens have allowed quite a bit of rubber at their goalies, which will lead to the several negative Corsi ratings you're about to see. Further a bunch of Penguins defensemen (Hal Gill, Rob Scuderi, Brooks Orpik, Mark Eaton) have the offensive instincts and put less shots on goal than the kids who play during intermission. To that point those four d-men have combined for 17 shots on goal in 31 man games....But hey, they've also as many goals (2) as Sidney Crosby and Alex Ovechkin do, so you can't knock them too much, the point being if you're not shooting and your partner doesn't shoot much, you're probably looking at a bad Corsi.
But who would have thought rookie Alex Goligoski's at the head of the class? And remember, Corsi only measures of even strength instances (click to enlarge)