This can mean several things. It can mean you were in a chat room and someone captured your IP address and is able to scan the internet and try to find you anytime you are on-line.
It can mean someone from an instant message site has captured your IP address and could do the same.
When you visit just about any website, there are logs that store the data, (cookies) from your computer. Technically, a webmaster could use this information to block you from their site or "flag" your computer so they can watch when you connect.
If you feel that someone is actually trying to "flag" your computer, you could use virtual browsers, like www.anonymouse.org
With this third party browser, your movements can not be tracked.
Flagging is not done by the average user, and not normally pursued by webmasters. (Too much traffic on most websites)
I hope this information is of some help. Thanks for the opportunity to serve...Thomas