The notepad...you mean that piece of school paper in the 2nd column on the forum page which shows all the titles of the threads? That's just a default icon.
<----is this the one you speak of?
When you start a new thread, you can CHOOSE which icon you want. Did you not recall seeing that when you composed your thread? There are 14 options (smilies, arrows, yellow yield signs, fingers pointing up or down and more.) You can find this section with the 14 options UNDER the TEXT box where you compose your thread and ABOVE the SUBMIT NEW THREAD button.
Is
the arrow pointing left on the post GREEN that you are talking about like this?
1).
Or is it like this?
2).
#1 = just one of those icons I told you about that you can choose to make your thread stand out from all the default notebook icons in the list.
#2 = you've posted something in that particular thread (like RedLeg505 mentioned).
In regard to
the YIELD signs...
If it was the red/white/black one:
This means you can report a problem in the post to a moderator. It's the REPORT POST button.
If it was the yellow one:
it was one of those fancy icons you can choose to make your thread more noticeable (when you're composing your post/thread). I mentioned it above...it's one of the 14 choices. People usually use the yellow one when they are posting alerts or what to call attention to that particular thread.