TOTP and HOTP
Time-based one-time passwords.
One-Time Passwords
A One-Time Password (OTP) is a code valid for a single login or a short window.
Two standard algorithms generate them: HOTP and TOTP.
The Shared Secret
Both algorithms start from a shared secret known only to the server and your authenticator app.
It is usually delivered as a QR code during setup and stored on both sides.