You can learn about it e.g. here: https://learnmeabitcoin.com/technical/keys/hd-wallets/mnemonic-seed/, maybe even start at https://learnmeabitcoin.com/technical/keys/hd-wallets/ or with Beginners stuff at that site.

Extending the mnemonic recovery words with an optional mnemonic passphrase is an extended feature and if you use it, you should understand it. Any unique optional mnemonic passphrase generates a new and unique wallet. It's imparativeimperative to be accurate, the smallest error or typo in the mnemonic passphrase gives you a different and empty wallet. Do not try to keep a complex mnemonic passphrase only in your wet brain memory. This is a rather sure recipe for later loss and desaster, your human memory will fail you (your mnemonic passphrase doesn't even need to be too complex, you likely will forget whatever you invented for it, especially after long periods of not using it).

And of course if you extend your mnemonic recovery words with a mnemonic passphrase, do not store both together. You need separate redundant backup storage for both.

You can learn about it e.g. here: https://learnmeabitcoin.com/technical/keys/hd-wallets/mnemonic-seed/, maybe even start at https://learnmeabitcoin.com/technical/keys/hd-wallets/ or with Beginners stuff at that site.

Extending the mnemonic recovery words with an optional mnemonic passphrase is an extended feature and if you use it, you should understand it. Any unique optional mnemonic passphrase generates a new and unique wallet. It's imparative to be accurate, the smallest error or typo in the mnemonic passphrase gives you a different and empty wallet.

And of course if you extend your mnemonic recovery words with a mnemonic passphrase, do not store both together. You need separate redundant backup storage for both.