What is a custom email domain?
Is a custom email domain just your main email?
A custom domain is a domain that you own. Imagine you have a personal email account for example name@lastname.com. You can now add it to your iCloud.
So it's not the same as your @me.com or @iCloud.com email address.
You can read more about it in this article:
Use Custom Email Domain with iCloud Mail - Apple Support
Personalise iCloud Mail with a custom email domain and share with family – Apple Support (UK)