An API key is a unique digital code that allows software applications to access specific data or services provided by another program, platform, or service. It acts as an identifier and a form of authentication, ensuring that only authorized users or applications can make requests to the API. While API keys help control access and monitor usage, they are not highly secure on their own and can be vulnerable if exposed or shared publicly. Proper management and protection of API keys are essential to prevent unauthorized access and potential security risks.