It's very simple and explained here (and also to the right side of the posting form). It will tag the member in your post and let them know that they were mentioned by you.
Sciameriken yours didn't work though cause you didn't wrap my nick (so it thought the comma was part of it and jumbled it in). If you click on my nick you'll see that it doesn't link to a profile.
Sciameriken I changed it to a BB function, with a [tag] button that appears on the posting form with the other functions. I figured this has better form and clearly delineates the username to avoid any confusions. Not sure why the original author didn't make it as such.