This is the regression variable, not to be confused with A, a general use variable.
On the TI-84, when a regression is performed, this is one of the variables that the regression results is stored in. The specific value depends on the regression performed. This is especially helpful when running a regression in a program so the results can be used.
:LinReg(ax+b) L1,L2 :a→A :LinReg(ax+b) L2,L1 :a→B :Disp A,"IS ONE OVER",B