Bip39. The passphrase just isn't selected from the predefined list of 2048 words – it can be any word or phrase you choose. Anyone else who discovers the phrase can steal the bitcoins. It has to be saved secret, for example it should not be typed into any website. Mnemonic https://yxzbookmarks.com/story19365887/bip39-https-linktr-ee-bip39-5-simple-statements-about-bip39-mnemonic-explained