You can optionally choose to get a Nostr Address (NIP-05) whereby a username at the domain referenced is mapped to your nostr account public key in hex format. Nostr Clients that support NIP-05 look up these values on the server for that domain referenced and verify that the public key for the username matches the one for your account. It's a way to get a "checkmark" but not required.

Incidentally, you can do this with any domain you control as well, and I've setup various NIP-05 nostr addresses using cornychat.com (mostly my bots, bot open to allocating on a case by case basis) and nodeyez.com (only contributors to that project, or some of my bots).

Services like nostrplebs let you pay for a username, and there are others out there that do it at different costs, or for free.