The secret code that users create in a stand-alone password manager application (not built into the browser) that is used to generate all the login passwords.
Like a Crypto Seed Phrase
The master password is similar in concept to the words created by a crypto wallet that serves as a master for all the keys that are subsequently created. The difference is that the master password is chosen by the user, whereas the crypto seed phrase is generated by the wallet. See
crypto seed phrase.