What does the star sign with number means?

The little number in the star is your reputation rating. If you hover over it, you will see a little tab saying 'myLot reputation: xx/100'. In this case, your reputation is in the 90's, so you're doing something right. You get this through people rating your discussions, and replies, so keep up the quality with those.

Addon to above replies : pls see more details about this Mylot user reputation system at a discussion started by Mylot administrating staff here : http://www.mylot.com/w/discussions/573319.aspx