Self-explanatory. When you hover the mouse over that icons, you see "New posts" and when you click, you will see all the threads that had a reply recently and you didn't read them yet. You can see this on the destination once click over the icon- "Recent Unread Topics"
This is a great feature but I doubt many use this feature. I haven't use this feature ever if I can remember.
Include me in those users who don't use this feature. Since I am browsing this forum through my smart phone then I couldn't hover a mouse. Well, I just click the button and find out that it is for recent unread posts. I thought it's symbolizes for "mute" since it looks like a speaker with a slash. Thank for the info but I prefer on browsing insidw of the thread than to use this.