It depends on the App.
If it's the App Store App, that means (as Jolly Giant suggests) that there are updates for some of your Apps. You can use the Wi-Fi to download those updates, or use your iTunes (for a quicker download, usually) to update the Apps in iTunes. The next
Sync will apply those updates to the Apps on the Touch.
For any other App, it really depends. I use a notes app *(Awesome Note +ToDo)* which puts a number at the top right corner to indicate Items Due. There's a "tally" (counting) App, which puts the tally number in that position.
Eventsbook indicates the number of events and birthdays due in the next few days. So, what your App does tells you what the number means.
Phil
Message was edited by: the fiend