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. BIP-39 provides a framework for generating that mnemonic phrase. Under BIP-39, standardized lists of words in different languages are provided and can be used for creating a mnemonic phrase. The English language word list has 2048 words and can be found here. The number of possible 24 word combinations of 2048 words exceeds the number of atoms on Earth!
For the Ethos Universal Wallet, the mnemonic phrase is 24 words (vs the 12 word standard used in many wallets). A single 256-bit seed, 24-word mnemonic provides master encryption for all of the private keys in each wallet you create.