What is a brain wallet?
Lisk uses brain wallets to create accounts and let users access them. That means your account has one passphrase, which grants you access. If someone else is typing in the exact same passphrase, they will gain access to your account. Your passphrase needs to be very long and secure to make it mathematically improbable that someone else will accidentally or willingly use the same one. Please note, if you lose access to your first passphrase you won't be able to recover your money.