Lol@joo is what people say when they're laughing.
Its like,
"haha at you."
The word "joo" is a typo of the word "You" and ever since someone made that mistake, joo became "you".
The @ simply represents "at", obviously.
The "Lol" means "laugh out loud"
Its sort of slang. A lot of scene kids say "Lol@joo". Look up scene if you don't know what scene is.
When somthing is really funny, people say it like "LOL@joo" or "LAWLS@JOO"