Project Soli is a sensor developed by Google that uses radar technology to detect finger movements and gestures. It is small, about 5x5mm, and can be integrated into wearables. The sensor captures submillimeter motions of fingers at a high rate of 10,000 frames per second. It determines hand properties using machine learning to translate gestures into commands. Potential applications include medical devices, gaming, and controlling gadgets through free-hand gestures without touching them.