If you want to check what's been going on, just tap the number and the notification tray will pop up showing you all the events that have been happening. A pretty useful feature if you disabled push notifications for your alerts so you don't have to get spammed by updates, yet you still know what's been going on.
One interesting thing about this new feature is that it was pushed out to Android devices before it made its way to the iOS platform - pretty strange, considering how Foursquare has always been rolling out its updates to iOS first. Could this be a new trend? Either way, you can head over to the Apple App Store to get the latest version of Foursquare for your iOS device.
source: Ubergizmo