An auth code (also called an EPP code or transfer authorization code) is a unique, registrar-generated code tied to a specific domain, required to authorize moving that domain to a different registrar. It functions as proof that whoever is requesting the transfer actually controls the domain.

Because possessing the code is effectively enough to initiate a transfer, an auth code should be treated with the same care as a password: never shared casually, and requested only when you're actually about to use it.