it’s an email generated by a service like Firefox Relay, Proton Pass or Simple Login, etc, connected to your email (only on the receiving part) used as a way to hide and protect your actual email from spam when registering to anything. That way, this alias email redirects incoming mail to your real email, and when you receive spam, or something you don’t want in your Inbox, you just deactivate that alias.
I think Proton bought SimpleLogin or something. It’s very similar, I use both to have more free aliases