I'm guessing by the French flag next to your name that you have the game set to French. So MY would be short for Moyenne or (Average in English).
I'm also going to guess that since your MY is 150 that all you play are coin games so it really means nothing for you unless you plan on venturing out of the coin rooms and trying out the many other ways of playing this game. If so then it will represent your total scoring average over 18 hole rounds.