Lean startup is a methodology for developing products and business models through a build-measure-learn process. It emphasizes experimentation to reduce wasted effort by validating hypotheses and pivoting as needed based on customer feedback. The key pillars are focusing on customer development and learning, continuously measuring product-market fit through minimum viable products, and only progressing when validation criteria are met to avoid wasting resources on the wrong solutions. The goal is to iterate quickly from initial ideas to a sustainable business model before running out of funding.