A HomeKit developer designs, implements, and optimizes applications for Apple's HomeKit platform, which allows users to control smart-home appliances using iOS devices. They write code, primarily in Swift, to integrate various smart devices such as lights, thermostats, or cameras into the HomeKit ecosystem. They ensure the applications are user-friendly, secure, and compatible with different Apple devices. Additionally, they troubleshoot issues, perform updates, and work on enhancing the functionality and performance of the applications. They also need to stay updated with Apple's guidelines and new updates to the HomeKit framework.