Your private key is the ultimate proof of ownership in Bitcoin — whoever controls it controls the funds. It is used to produce digital signatures that authorise spending, without ever being revealed on the blockchain. This is why the phrase "not your keys, not your coins" is so central to crypto: leaving funds on an exchange means trusting that exchange with your keys. Private keys should never be shared, photographed, or stored on internet-connected devices, and are best backed up via a securely stored seed phrase.