A mnemonic phrase is a human readable encrypted private key. It is usually used to derive multiple private keys. It is a 12/15/18/21/24 words phrase that can help you access an infinite amount of accounts.
Mnemonic phrases are used by Tokenpocket, Ledger, TREZOR, MetaMask, Jaxx and many other wallets. Each of these examples is “paths” and the account you access is linked to the mnemonic phrase (its encrypted private key). If you select a different path it will generate a different address.
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.