From Facebook Developer Wiki

Description

We are creating a new channel to enable applications to directly notify their users that they should take an action within an application. The Counter will be displayed next to an application's bookmark on the Home page, and will likely be located beside all applications on the Application and Games Dashboards as well.

Estimated Timing

November / December 2009

Details

You will have the opportunity to increment your Counter to indicate to a user that they should take an action in your application -- for example, take their turn in a game, or see a comment from another user on content they created within that application. The Counter can be incremented by your application, and when a user clicks through to the application, the Counter will be reset to zero.

We'll continue to update these mocks as we refine them. (For example, we are still designing exactly how Counters will be displayed beside non-bookmarked apps. Stay tuned.)