I want to learned about Passphrase related to lock or unlock wallet.dat. Sorry for not clarifying my question.
Well it's actually the password you used when you encrypted your wallet file and that's pretty much it. It could be a random word/group of words/mnemonic and it entirely depends on you on what would that passphrase be. Your passphrase is like the 'master keys' needed in order to access the wallet and its contents, together with its balances and, well, totally full control of the wallet. It's a must to have such a strong passphrase in order to not get hacked by nefarious hackers around as it serves as another layer of protection for you and your money.