A private key is a secret number used in cryptography to secure digital information. It is part of a key pair, which also includes a public key. While the public key can be shared with anyone to encrypt messages, the private key must be kept confidential, as it is used to decrypt those messages and sign transactions. In the context of blockchain technology, private keys are essential for managing digital assets like cryptocurrencies. If someone gains access to your private key, they can control your assets, making it crucial to store it safely, often in a secure wallet or hardware device.