A private key is an encrypted key, a 256-bit number that can be represented in different ways to spare users the need to remember long random strings of characters. 

The mnemonic associated with your private key is a back-up key that enables restoring your wallet if needed. The mnemonic is simply a list of 12 to 24 random words that are easier to store or remember than the private key’s original format.
The mnemonic is generated only once, in the wallet creation process, and is the human-readable version of your private key. It can never be modified nor recovered if lost. 

As the wallet owner, you are the only person responsible for your wallet’s access credentials, so it is crucial to keep both your password and mnemonic safe and private. It is technologically impossible to recover your wallet and the funds it contains without the password and mnemonic. It is also impossible for thieves to break into your wallet unless they have access to your password or mnemonic.