‘Mnemonic’ refers to any system that helps you remember something. It could be a series of rhyming words, abbreviations, a song, or even a journey through an imaginary place. When creating a cryptocurrency wallet, a mnemonic phrase is sometimes generated in order to provide an easier way to remember your private key, instead of having to memorize an alphanumeric string. 

Also referred to as a ‘seed phrase’, it’s important to keep it somewhere safe. As with any private key you hold, if lost it means you’ll also lose access to your cryptocurrency. Try to keep your seed phrase somewhere it can't be destroyed or stolen. 

If possible try to keep them separate from other details about your crypto-wallet. To be highly secure, keeping your passwords away from internet connected devices is also advised.