Lean Startup, according to its creator Eric Ries, is an approach to launching a new business based on learning, experimentation and constant iteration with customers.
This concept, applied to reality, allows companies to improve their products or services or to create new business lines through a constant improvement cycle that allows them to quickly and economically verify the validity of their model and the necessary improvements required by end users.
Everything starts with the definition of hypotheses about the new product and then proceeds to test them with the users through experiments.
Then, using the information obtained from these experiments, the hypotheses are refined through the learning achieved to move towards the development of a successful product.
All of the above can be summarised simply in three steps: define, test and measure, analyse results and learn, back to step one.
Knowing which metrics are most important at each stage of the product is what will make the difference between success and failure.