For a more in-depth analysis of what PDO is, this article by Cam Charron is a nice start. I like PDO because it's easy to calculate, it's a really interesting calculation of "luck" and it's also really funny when a team who has really good or bad "luck" starts regressing to the mean (see: last year's Minnesota Wild). The "league average" is 1000 - your save % and your shooting % at even strength on average should create a number close to 1.
From earlier, here's the Hurricanes' PDO chart (averaged over every five games) (Thanks again to Shutdown Line for creating this chart!)
The Hurricanes started out around average, and now they are really bad! The thing is - the Hurricanes aren't very likely to rebound. There are ten games left, and neither Dan Ellis nor Justin Peters is a standout goalie likely to suddenly ramp up his save percentage. Sure, they might steal a game here or there, but nothing that, combined with the Hurricanes' mediocre goal-scoring ability right now, is going to shoot that PDO through the roof.
There's only so far luck can carry you, especially with bad goaltending.
Now for everyone calling in to hate on Tuukka Rask or Anton Khudobin or whatever else have you, let's take a look at the same chart for the Bruins:
Oh look at that! The line wiggles a bunch, but it wiggles around that 1000 mark - up for five, down for five. It never deviates too far above 1000 - indeed, the highest "five" was between games 20 and 25, and that's because it included a game where Boston scored four goals on 21 shots 5 vs. 5 against Toronto. Conversely, it never even dips as far down as 950 - because OH HEY, Boston has consistently good goaltending at even strength in combination with a pretty average shooting percentage. In fact, if anything, the Bruins have a slightly lower shooting percentage at even strength, which highlights the fact that our goaltending is actually REALLY consistently good. Suck it, haters.
Meanwhile, if you take another look at the Canes' chart, it's obvious that losing Cam Ward really sucks - their PDO was above 1050 until he got hurt, and while it was likely to regress anyways, without Ward it's now down below 900. Yikes. That sucks.
For the record, right now through 37 games the Bruins' PDO is 1004. Above average. Aw yeah.