That's a great question - it's so easy for us to take for granted what everything does since we see it every day.
That's the subscription icon. When you see a forum thread, a user, a product, or a wiki that you find interesting, you'll notice there are "Subscribe" buttons on all of those pages.
If you choose to subscribe to something, whenever it gets materially updated (e.g. a user that you're subscribed to writes a new review or creates a wiki article) it will register as a subscription update. On your home page (when you click the Green Home logo or the Home Tab while logged in) you get a running digest of all subscriptions that have been updated since you last logged in. You can also receive these updates via email in real time, daily, or weekly.
The idea is that when you log in, you're immediately presented with what's new in the areas that you care most about so you don't have to dig around quite so much.
We would REALLY appreciate any feedback and/or additional questions on the subscriptions system. Let us know how to improve! Thanks,
(workin at Huddler)