Also referred to as a recovery phrase or seed phrase, a mnemonic is typically a sequence of 12 or 24 randomly generated words used to create and recover a Bitcoin wallet. The mnemonic encodes the random entropy your keys are derived from, in the form of easily legible and rememberable (thus more human-friendly) English words. It is used as a backup recovery plan to access your bitcoin, as outlined in BIP 39.
It is essential to keep your mnemonic secret and to follow best wallet practices for optimal safekeeping. If compromised, your bitcoin may be subject to theft.