An API plays a critical role in mobile app development by providing a set of protocols and tools that enable communication between different software components. APIs allow mobile applications to interact with external services, databases, and other applications, facilitating data exchange and enhancing functionality. For example, a mobile app can use an API to retrieve data from a web server, integrate social media services, or access hardware features like GPS. This modularity and scalability make APIs essential for modern app development, enabling developers to create rich and connected user experiences.