iTunes account

Read this support articcle (it will be a lot prettier than my copy and paste here ;-) )

Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

What you need to know about authorizing your computer

• You can authorize up to five computers at a time to play your iTunes Store purchases.
• Each computer that you authorize counts as one authorization — even if you authorize the same computer multiple times.
• If you have five authorized computers and want to authorize a new computer, you have to deauthorize one of your computers.
• You can only authorize a computer that you have access to. You can't
• authorize a computer using another computer or an iPhone, iPad, or iPod
• touch.
• You can authorize your Mac in the Apple Music app, Apple TV app, or
• Apple Books app. Once you authorize your Mac in one of these apps, you
• can access all of your iTunes Store purchases.

How to authorize or deauthorize your Mac

1. Open the Apple Music app or Apple TV app. In the menu bar, choose
2. Account and make sure that you're signed in with your Apple ID.
3. Choose Account > Authorizations, then choose Authorize This Computer or Deauthorize This Computer.
4. 
   
5. 
6. Sign in with your Apple ID and password. Forgot your Apple ID?
7. Click Authorize or Deauthorize.