Its a nip-05 or "nostr address" It's a little file you host on a web server. There are several services that do this for you Or if you have a domain that you would like to use, you can do it yourself.
Discussion
ok, what service should I use?
whichever one you want. I host one on my server but its paid on sovbit.host
Took me a sec to find it but this page has a good list of free and paid ones.
I think that page also has how to do it yourself if you want.