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 allow them to send encrypted messages, the private key must be kept confidential by the owner to maintain security. In the context of cryptocurrencies like Bitcoin, the private key allows users to access and manage their funds. If someone gains access to your private key, they can control your assets, making it crucial to store it safely and securely, often in a digital wallet or hardware device.