it means fuck off thats all it means its plain and simple and used when your words are censored like in online chatrooms and gaming and in public places like school.
f off and die biatch translates to fuck off and die bitch the "bad" words are replaced or omitted so you can get you point across when someone pisses you off.
Example
1.
"You ruined my car on purpose so why dont you go f off!" (fuck off)
2.
person1: "your typing sucks today"
person2: "f off, loser"
person1: " my aren't we touchy...."