It means that the thread has "tags." These are mostly to make it easier to search for old threads. For example, if I wanted to read some science-fiction stories, I could go into advanced search and type in "Science fiction" in the tag search bar. Then all the stories where the original poster added the tag "science fiction" would pop up.
I think funkybassmannick is right. Agatha Christie could have meant the "Edit Post" button, and that would seem the most accurately described as a pen, but I'd discount it as being too obvious since it's labelled "Edit Post". I think the tag symbol is the more likely, but it would be helpful if Agatha Christie could give a little more information.
No, I dont mean the edit button or any of the buttons at the bottom right of the posts. When I go to a particular section, before I actually click on a thread. when I look to the right there is sometimes a small blue pencil, but not against all the threads. This is before I can actually read the varioius posts on a particular topic or title. Not sure if I've made myself clear.
It means one or more posts in the thread have attachments. Pause your mouse over the icon and you will see a small window pop up saying how many attachments in the thread.