The mnemonic is used to derive multiple private keys. It is a unique wallet phrase that gives you a human readable format of words to backup your wallet for recovery. It is typically a phrase that allows you to access an infinite number of accounts. At its purest form it is a pattern of letters, words or associations that help you remember the information easily. It is used by Ledger, TREZOR, MetaMask, Jaxx and others.
The accounts you can access with this phrase are determined by the technical "path". imToken allows you to import a wallet using the mnemonic from different paths. However, you cannot export a private key on imToken.
Here is an example of what it should look like:
scissors invite lock maple supreme raw rapid void congress muscle digital elegant little brisk hair mango congress clump
Remember: We do not save your wallet password and mnemonic in our server, once forgotten, it cannot be recovered.