The little lightbulb by a post, what does it mean?

I am not even sure if it is a light bulb... But if you look at the forum, and at a certain thread, you will see the # of replies (I think that is what it is), and then a little light bulb thing with a + number beside it. Not all of the threads seem to have this, but some do. I was thinking maybe it had something to do with the # of replies, but after viewing a few threads, that doesn't really seem to make sense.