A "triple 9" (aka 999 code) over police radio means an officer is down. When an officer is down, all cops must rush to respond. This means if you are willing to take that step for the perfect heist, you would take down a cop in one part of the city. This, in turn, would suddenly open up all the time in the world to commit the perfect crime on the other side of town.
This explanation comes clear about halfway through Triple 9. Up until that point, there have been a plethora of "character" introductions, aka people introduced with no character. Some are good cops (Casey Affleck), some are bad cops (Anthony Mackie, Clifton Collins, Jr.), some are not cops at all (Norman Reedus and Aaron Paul... at least I don't think they were cops). If they are a woman, they are a hooker, or a crackhead, or a pantless wife. If they are lucky, they might be Kate Winslet, who plays a Russian mafia wife and sort of sister-in-law to one of the bad guys. Kate works on a wobbly Russian accent that gets more pronounced when she verbally relishes the name of her husband Vasily, who is in prison in Israel (or something). She demands a seemingly impossible heist, with the threat of turning one of the men, and his son, into another Kosher meat product that her factory produces.
There is an extended sequence in the middle of the movie involving a police raid on a housing project with Mexican gang members. This goes on and on, is kind of well staged, and ends up having little to no effect on the rest of the entire movie. It's just kind of dropped, like the three heads of Mexican gang members that show up on the hood of a car. Who are these guys? Does it matter? No. Moving on!
There is a bank heist. There is a foot blown off with a bomb. There is at least one dead girlfriend (wait... were we supposed to know her?). There are uneven sound effects and one strangely lame car explosion. Meanwhile, Oscar-nominee Chiwetel Ejiofor tries to say "a'ight" with a straight face, Casey Affleck has a weird case of hat-head, and Woody Harrelson shows up like he has popped in a fake set of teeth and careened out of True Detective outtakes.
None of it makes any sense. So much so, that I actually started getting angry. "What the hell is this movie about?" I said to my friend, incredulous, only a third or so through the film. I have no idea. I saw more than one person walk out of the theater, giving up. I wanted to walk out, too, but I figured it was almost over. It wasn't. It went on for another half hour. Sigh. What a waste. What an astonishing waste of talent in one mess of a film.