What does Starter Digital (Automatic Renewal) (Monthly) get me?

Because you have a subscription you purchased from a 3rd party app on the App Store. This is NOT an Apple product. You are just paying for it via the only way a subscription can be paid. In other words, Apple has nothing to do with whatever this subscription is other than hosting it in the App Store. You need to address your question to the App Developer. I've never even heard of whatever Starter Digital is, to be honest.

I have no clue what Starter Digital is, but if you have a subscription to it, it's a 3rd party app, so you should ask the app developer about what you're paying for.

Are you sure this message is real and really from Apple and not one of the legions of scam messages around?

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

If you want to cancel your subscription, see this link --> See or cancel your subscriptions - Apple Support

I thought it might be a cable TV subscription at least until I read further - at least around these parts, Comcast used to call their cheapest plan exactly that.

https://www.cabletv.com/xfinity/channel-lineup/starter

I tried to find an app called Digital Starter in the App Store, but came up empty handed. I have no clue what the OP is paying for.

The OP said they are paying for it and get an invoice from Apple and the payment is paid through their credit card.