Those are the (male) divisions you can play in a PDGA tournament:
-MA3: Men's Amateur 3 or Recreational
-MA2: Men't Amateur 2 or Intermediate
-MA1: Men's Amateur 1 or Advanced
-MPO: Men's Pro Open (I think that's the right definition) or, simply, Open
There are others for the Masters and Junior divisions as well. Women's divisions are the same as men's simply swap the "M" for "F," so women's pro would be FPO.