What does Diablo III a 1a2 mean?

This is the tradition of the Diablo series.

Like a different scene in a play. Diablo 3 is divided into four acts, each of which follows the plot to a different scene.

ACT means "performance" in drama. Diablo will use ACT 1 2 3 4 to divide the plot.

So we call these four-act games A 1 A2 A3 A4 for short.