In simple

Shared secret key + Time = token (which we see in the authenticator app)

1. Shared secret key will be created in server and shared to client i.e (authenticator apps).
2. Authenticator app will create a token from time and shared secret key.
3. Also shared secret will be stored in database.
4. While checking server will create a token from time and shared secret key (server will get it from DB) and it will check the result with the token which server received as a part of request.