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

The Dashboard APIs (including the ability to set Counters) are now available for testing. Counters will launch to users in February 2010.

Note: The Facebook Dashboard API is now available. We have sandboxes available for the Applications and Games Dashboards. See our blog post with details about the new APIs, including how to set Counters.

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.