What is a private key?

A private key is randomly generated to unlock a string of characters corresponding to a wallet.
During transactions, a private key is used to generate the signature necessary for the transaction to prove ownership of the asset. If anyone gets hold of your private key, enters it into a wallet, and sets a password (they do not have to have your original password), they can enter your wallet and have full control over it, compromising your digital assets.
Therefore, you must keep your private key safe.