Many applications and games have In-App purchases. In-App purchasing lets you buy items within an application or game. Some of these items will unlock additional features for an application or game. For a complete breakdown on the types of in app purchases, visit the Amazon help page: In App Purchasing 101.
It’s very easy, especially with some of the more addictive games, to get in over your head and spend a lot of money. The ability to one click buy items without ever leaving an app, compounds the problem. I disable in app purchases. That extra step helps me decide if the in app purchase I’m thinking about buying is really worth it or not. It makes it less of an impulse buy. Personally, I have far less buyer’s remorse and sometimes I learn a bit more about the in app purchase before making it. This is also useful if you loan out or let someone else use your Kindle.
To disable In-App purchase, go to settings then apps and uncheck the box next to In-App Purchasing.