It is either when someone has posted a picture on the thread but used the large size.
It also happens if someone posts a link to another web site and the link is very long, because for it to be clickable to open it must be in one line.
This is a shortened link to a lamp on the B&Q site: http://tinyurl.com/2guur4
If I had used the full link that was in the address bar there are 270 characters in it so it would go right off the screen and you would have to scroll across.
you can also use the code [url=http://www.diynot.com/forums]Index[/url], in this instance only the short word appears on the screen instead of the longer url i.e.Index