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.