That happens to me a lot when pages are loading. I wish it would stay, it's pretty useful (It tells us mod stuff to on applicable threads)
I've never noticed this before, but turns out it's just a caption. Hover your mouse over the icon (that should be there instead of this text) next to topic you have posts it and it should appear after a sec
If you are talking about the downward arrow, when you click on it, it takes you to the first unread post in that thread... ...at least that's how it works for me.