Find out what the purchase is for:
View your purchase history for the App Store and other Apple media services - Apple Support
As for preventing charges, it will depend on how it occurred in the first place. Apple is not deducting money without authorization. For purchases on your device, it requires double clicking the side button with the amount showing on the screen, followed by verification with Face ID or Passcode. There are situations where that authorization could have occurred, such as:
- Anyone that knows your passcode is authorized to make purchases on your device.
- Anyone that knows your Apple ID Password can login to a device and make purchases.
- If you authorized a free trial, that same authorization screen also shows the subscription price if you do not cancel the trial at least 24 hours before the trial period ends.
- If you are the Organizer of a Family Plan using Purchase Sharing, then you have authorized other family members to make purchases.
Once you are able to see what was purchased, it may help you determine how it occurred, and then we can offer specific tools such as Screen Time restrictions or a Passcode change to prevent it from happening again.