What is the difference between bitcoin wallet passphrase and private key? Which one is more important?
A wallet contains one or more private key (and the associated addresses). A private key is what allows you to spend the bitcoins at that address. Many people use the term "wallet" incorrectly when they mean "address".
A wallet passphrase is simply the passphrase used to encrypt the wallet. If the wallet is not encrypted then anyone that gains access to the wallet can copy the private keys it contains and steal your bitcoins.