A deterministic wallet is a system of deriving keys from a single starting point known as a seed. The seed allows a user to easily back up and restore a wallet without needing any other information and can in some cases allow the creation of public addresses without the knowledge of the private key.
Source and additional details: https://en.bitcoin.it/wiki/Deterministic_wallet
This means that your addresses can be derived from your 12-word passphrase.
If you unintentionally change (or misremember) your wallet passphrase, you may “successfully” login and discover that your wallet is “empty”, but that would be a new wallet you just created by using a different passphrase.