Bubble is a programming tool that allows users to rapidly build and test ideas by writing "code" using a click-and-draw interface, cutting down on time and learning curve. As a UX designer, the author struggled with prototyping applications but found Bubble to be a game-changer for validating design decisions with real user data. The author built a reading tracking app called Book Keeper in just 6 hours of work, which uses the Google Books API, repeat groups, and workflows to display user data in a structured way, allowing users to input book titles, track page locations and notes, and access their data even when logged out.