The title might seem pretty self-explanatory: girl forms a group of thieves, creates a heist society. Even though Kat and her crew don't refer to themselves as a "heist society," they could accurately do so, were they so inclined.
However, Ally Carter's titles are always a little goofy, and they often contain puns. Check out these titles from her Gallagher Girls series: Cross My Heart and Hope to Spy and Don't Judge a Girl By Her Cover.
So Heist Society is a clever little pun, too, this time on the phrase "high society": "a category of people deemed to have greater social status or prestige." Unless you somehow happen to be a jet-setting art thief, Kat and her family definitely fall into that category for you.
However, Kat and co. got that way by hook or by crook. We have a feeling that they weren't always members of the "high society"—and really, they're still not, if you think about it. They have to keep their jobs and careers a secret.
When all is said and done, the Bishops and their thieving friends are still kind of on the outside, fighting their way in. We guess the moral is: if you can't beat 'em, steal from 'em. (Um, no. We don't actually recommend that, sorry, Shmoopers.)