I'm getting items with a certain amount of stat "Hate gain" on them but I don't know what hate even does.
makes one's attacks generate more "hate" which for a tank, is best to have maxed at 100%, so as to keep the beasties' attention OFF your group members,...
If you're not a tank, you don't want hate gain. You especially don't want it if you are a mage and wear cloth armour (very squishy).
It can be confusing if you are a newer player, here is a link for some additional information that might bring it all together better for you. Simply put, if you are tanking you want more, if you are not a tank you want less. http://eq2library.com/guides/stats-breakdown/