Mnemonic phrases are another form of private keys, while private keys represent sole ownership of crypto assets, meaning that anyone, who gets hold of your private keys, will have full control over your assets.
To provide users with an easier way to memorize complex private keys and manage crypto assets, the developers use a certain algorithm to convert your 64-digit private key into a set of common English words or Chinese characters, i.e. mnemonic phrases. Usually, mnemonic phrases consist of 12, 15, 18, or 21 words taken from a fixed word pool, whose generation sequence is based on a particular algorithm.
Once lost, the mnemonic phrases CANNOT be recovered. Please make sure to back up your mnemonic phrases properly and keep them safe.
Check out the tutorial here: How to Back up Mnemonic Phrases