A mnemonic phrase, mnemonic seed, or seed phrase is a group of words which is used to generate a master seed which is then used to generate a master key.
This Master key / Mnemonic Key ( recovery / backup phrase) is the key to access a Crypto Currency Hot Wallet.
The Seed phrase can for ex. look something like this:
hat – umbrella – tree – variety
guitar – clean – professional – function
one – travel – flower – happy
running – car – pharmacy – pillow
Often more than 12 words are needed (for extra security). The words has to be entered in their specific order to gain access to the wallet.
It is advisable not to have these words stored on a phone, computer or other devise but to write them down on a piece of paper or booklet and keep a copy it in a fireproof safe.