Actually "Haters gonna hate" said as "hatersgonhate" but meaning haters are going to hate. Used when someone does something someone dislikes are gives that person crap about.
it means what it means-
HATERS ARE GOING TO HATE
Joey: Dude i just got mw3. Mad chill bro, you have no idea.
Martin: What the fuck? Dude that game sucks dick.
Joey: Whatever man, haters gon hate.
A phrase commonly used by the African-American society. Typically they use it after someone has hated upon something they have, even though there is every reason in the world to hate it.
Also, these people tend to think that themselves, opinions, and their possessions are far more superior than everyone else, and that everyone else is wrong.
Also see: A "Know It All"
Dan: Dude look at your car, its a under-powered piece of shit!
Tyrone: Are 'yall serious riite now mayne!? Dis whip handles amazing and wait for these pieces I 'bout to buy.
Dan: Still a piece of shit. Always will be. Go fuck yourself.
Tyrone: 'Yall mayne! Haters gon hate!!