A passphrase is an extra layer of security for your wallet, it is an extra 'word' added to your seed phrase which creates another wallet and keys for you, take note that your passphrase does not have to be a 'word', you can use and add whatever you want, even characters to make it stronger and hard to brute force. So whenever you use your seed phrase without adding that passphrase you get the base wallet, but when you use your seed phrase + passphrase you get your second wallet, this set up is great for plausible deniability.

So with a passphrase set, if an attacker compromises your seed phrase, they will still need the passphrase to access your funds. Take note that if you use a seed phrase + passphrase set up, you will have more backups and you have to keep them in separate locations to avoid the possibility of a single point of failure.