The Lean Startup method teaches companies to test assumptions through experiments, measure progress, and make decisions based on data in order to reduce uncertainty and build sustainable businesses. It draws from lean manufacturing, agile software development, and customer development approaches. Companies use tools like business model canvases, customer interviews, and minimum viable products to test problems and solutions with customers, iterate quickly, and pivot if needed based on experimental results. The goal is to find product-market fit and scale efficiently before running out of resources.