Moonlight Client is an open-source game streaming client that allows users to stream PC games from a gaming PC to other devices such as Android phones, tablets, and other computers. It leverages the technology of NVIDIA GameStream to enable low-latency streaming of games over a local network or the internet, provided both devices have a compatible NVIDIA GPU. Moonlight Client supports various features such as HD streaming, controller support, and customization options for streaming quality and performance.