Also called a “seed phrase” a Mnemonic phrase is a secret combination of 12 words that is created at the same time a wallet is and acts as an identifier to that wallet. The purpose of this 12 word combination is to act as a backup in case the device on which a wallet exist is stolen, broken or otherwise compromised. Using the 12 word sequence a user can recover their wallets & crypto assets on a new device. This phrase is typically re-written and stored on a piece of paper is a very safe place.
If anybody gets your mnemonic phrase they can steal your wallet & assets! Do not share the phrase with anybody.