A Libav developer works on the Libav project, which is a set of open-source libraries and tools for handling multimedia data. They develop and maintain software components for encoding, decoding, transcoding, streaming, and playing audio and video data. This involves writing, testing, and debugging code, often in C language. They also work on implementing new features or improving existing ones, enhancing software performance, and fixing bugs. Additionally, they collaborate with other developers, contribute to documentation, and may also provide support to users. Their work is crucial in providing a foundation for various multimedia applications.