What does the star next to username mean?

The number in the star is your reputation. It decides how you will get paid. If you don't believe me, ask Mike. Mike knows.

I'm sure it's good like having a star credited on our works at school. But i was thinking why i don't get a green star and jump it to silver star with number 7 on it...

I echo what everyone who posted before me has said. It's your "clout" as far as how well your posts have been received (supposedly) by other MyLot members. I would also add to not worry too much about the star rating and just post. I have seen people's star ratings go down just because a lot of people gave their posts negative ratings based on their agreement with the post rather than the quality and there have been other people who have been unfairly rated by other members because their English was not up to par (which was understandable in most cases as it was not the poster's first language). So in truth you star rating could potentially go down for stupid or unfair reasons. Focus more on whether your posts are worthy of positive reception, rather than whether you seem to actually be getting it. Good luck!