Backup codes are one-time codes (alphanumeric, 10-character in length) that a MX user can use instead of the 6-digit numeric code generated by an authentication app.
When you first setup MFA, you will be prompted to record a copy of the three Backup codes that have been generated.
Where to use Backup codes
These codes are ideal when you do not have access to your authenticator app (say you have lost or just forgotten your phone).
Check / create backup codes
You can check which codes are live / expired and if you have used all of your codes (or suspect an information breach) you can create new backup codes at any time.