In-App Purchases (sometimes shortened to IAPs) are optional purchases directly within an app that you have already downloaded. Although commonly found in apps that are free to download initially, some paid-for apps also contain In-App Purchases as well. They can be used to buy extra items, unlock more features, subscribe to a service and much more. Some apps make In-App Purchases very clear but with others there can be more ambiguity. However, the tablet settings allow you to control how In-App Purchases are managed in all the apps on your device, to prevent unwanted or unintentional purchases. 

Please refer to the Google Play store website for instructions on how to restrict In-App purchases on your Android tablet.