Here is an explanation video provided by Apple explaining what an Apple Account is.
What is an Apple Account? - Apple Support
You can see if you have an Apple Account on your Mac by going to the Apple menu in the upper left corner, then going down to System Settings. The Apple Account would appear on the top there and if one has not been entered, you will see a prompt to Sign In. It is also the same one that is used to sign in here. It is a single account that can be used for all Apple services.
For many services on your Mac, such as using iCloud, Messages, App Store, etc, the Apple Account would be needed. It does not help you to understand your iMac, but this User Guide may be able to help. The slight problem is that the iMac may not have all the features in the User Guide since the iMac is using MacOS 12.7 and likely is not capable of upgrading any further to take advantage of those features.
Welcome to iMac Getting Started Guide - Apple Support