It's exactly what it sounds like. It shows how the tackles were distributed in a game or season. I mocked these up to compare past defenses and where the most tackling too place. (I haven't done one for 2011 yet, though).
In 2012, the tackles were spread pretty evenly among the linebackers, and secondary, with Rush DE getting quite a few (Damontre Moore) and the rest of the line logging decent tackles. The Rice game actually compares quite favorably to this distribution of tackles, except with far fewer TFL, meaning our LBers were chasing guys down from behind. (not good).
I decided to go ahead and dig up an old Sherman / DeRuyter Era distribution as well for comparisons sake (2009-2010)