It means that person has been a member for less than 14 days. In fact, if you mouse over it you can see how long they have been a member for.
It's purpose is to highlight new members so you can all make them feel welcome.
Please send them a PM or put a comment on their profile becuase a welcomed member is a member that will come back to the site again and really join the community.
THANKS
(Don't forget you can also search specifically for new members in the advanced profile search)