An invoice from apple and I don’t know what it is for

If you're sure that that charge comes from Apple (check your bank statement very carefully), start reading this support page:--> Subscriptions and Billing – Official Apple Support, and check each of the first 3-4 sections.

If you cannot find the information you were looking for, please contact iTunes Store Support Team directly, using this specific link:--> Apple Support. If your problem is not among the options shown there, describe your issue, providing as much detail as you can.

Regards

Giulio

You also need to check if the invoice is REALLY from Apple.

Some phishing emails are sending emails to you, let you know that you have purchased something, and if you have questions, please contact Apple, blabla... Their login page is also looks like original one. Proceed with caution.