well the first one is simply how much you have won total. Then the actual amount you have won is the net figure after you factor in rake and where you finish in the tournament.
The $ev is a bit misleading and not really great unless you play mainly hypers or STT games.
The last part is pretty big question to answer. Your best bet is to get into the HEM forums and look around there. But in general, you can use it to view your own personal stats, work out how much you are making in certain positions at the table, this helps to pinpoint leaks in your game. You can also use it to work out how others are playing at your table, which is really handy if you have a few thousand hand samples on players.
In regards to what stats are important, that comes down to knowing how to use them and experiance as to what types of ranges you should be playing in certain positions, i could be here all day trying to explain that one, that will come with experiance and working on using the program and getting better at poker, but if you have any other questions, Holdem manager has it's own thread which i'll link you too shortly, plus they also have a forum which might have some videos on it to help get you started.