The fill tool button has been present for a while, but it was greyed out. We teased it in an effort to make our users think that releasing it was just around the corner. The truth is that this was a "hard nut to crack" (one of Ben's favorite expressions used to refrain for becoming extremely vulgar when it comes to not having easy wins against his code).
So essentially, the fill tool fills stuff. In Mandalagaba, we define stuff as being any surface delimited by pen strokes. A user can draw about anything and as long as there are stokes defining areas with closed borders, he or she can fill that area with any color.