The twelve word mnemonic phrase is a recovery phrase, also sometimes called a seed phrase or backup phrase. Knowledge of this phrase gives anyone control over all money in a typical wallet. The purpose of this phrase is to allow you to re-create your wallet if, for example, you lose, damage or destroy the device on which it is installed.
You must keep your recovery phrase secret.
From the recovery phrase a new wallet installation can generate a seed number and a master private key. From that one number, a Hierarchical Deterministic (HD) wallet can generate further private and public key pairsas needed. All of these individual private keys also need to be kept secret.
Examples