Prototyping mobile applications involves creating early versions of ideas to explore alternatives, visualize concepts, and test with users. Good prototypes are quick and cheap to make, with minimal functionality but enough to test key tasks and user flows. The prototyping process involves planning user stories and flows, building mockups using tools like Balsamiq or paper prototypes, testing prototypes with users, and refining based on feedback before sharing ideas more broadly. Prototypes are valuable for answering questions, failing ideas early, and generating new insights compared to static pictures alone.