User retention in mobile or web apps is crucial for success. Key metrics to measure include user retention rate, customer churn rate, daily/monthly active users, and customer lifetime value (CLV). To improve retention, meet user expectations by setting clear ones, aligning with UI/UX best practices, reducing app friction, making onboarding seamless, incentivizing return visits through gamification or loyalty programs, increasing personalization, and avoiding being annoying. Building an app with no-code tools like Bubble can help in faster iteration and improvement based on user feedback.