There are many ways to start a company and the traditional approach has been to complete the business plan, have all the ideas and business plan and then jump in. However, with the lean startup methodology the idea is to fail often and quickly before the company invests too many of its resources. The idea of selling before a product is made is one of the most troubling features of this method but the basis of the method is to develop something only if it sells. Otherwise, not bother.
Read more about it in the Harvard Business Review article below.