Text is the default symbol for a thread.
A thread icon is red if the thread contains unread (by you) posts and grey if you're up to date. This functionality seems to break if you go and read a really old thread.
A flame ("this thread is on fire") is if a thread has gotten more than 20 (IIRC, it's something on that order) responses. With the old/previous theme, the text icon appeared to be scrolling instead.
The "light bulb" is actually an exclamation mark and means that the topic is sticky (and therefore listed on top of a subforum). I make things like logs sticky so they're easy to find.
The "i" (for information) is an announcement. These would appear at the top of each subforum on the board index. All the forum rules are "announcements". A thread can also be global. I don't think that global gets its own icon but it means that the thread will exist in each and every subforum beyond where it was originally posted. The rules are global announcements.
A padlock means that the thread is locked.
It used to be that people could pick other icons when starting a thread, but I removed that option because I didn't like the look of it.
dumb question
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Jason
Re: dumb question
I thought they were visual indices of Daylen's higher brain functions that he telepathically communicated while tripping balls on acid.