A one time code is a secure and temporary code sent to your email address when you log in to My Account. This means you’ll never need to remember (or reset!) a password.
The one time code is only valid for 5 minutes after they're sent and will expire shortly thereafter and you will have to request a new code.
The code is considered as a secure login method, as it checks that the person trying to login to your My Account has access to the registered email.
Learn how to login in using a one time code.