In cryptography, messages are secured by a combination of public and private keys. Your crypto private key is a randomly generated string of letters and numbers, or sometimes in the form of a 12/24 word seed phrase, that acts as a password to your crypto wallet. It is used in a cryptocurrency transaction for proof of ownership. There is never a valid reason to share your private key or seed phrase with anybody. It should be defended at all costs because with your private key, anybody can take all your funds.