Each new Bitcoin address comes with a corresponding private key which is required to transfer coins on the former. Private keys are long strings with 64 characters in the range 0-9 or A-F, which makes them a challenging to remember. For this reason, private keys are typically stored on medium like a hard drive or a piece of paper. An alternative for the previous is creating a private key from a passphrase. The latter is done to create a so-called brain wallet, because the passphrase should simply be memorized. Passphrases should, however, be sufficiently long and difficult to guess. Some tricks like character substitutions and misspellings can be applied, making it harder to algorithms to recreate the passphrase. If done correctly it is still not as secure as a perfectly random private key, but at least there is no risk of losing the keys (except for forgetting the passphrases).