START - get out of the building
The target of this first phase is to iterate from a rough idea towards a first, testable business model. Central elements are getting to know your customers and to verifying whether you're dealing with a big enough problem to solve.
SELL - make something people love
The second phase is about maximizing your learning by creating a minimal viable product and selling it to actual customers. It's about creating a small but returning customer base who loves your product as much as you do.
SCALE - turn your growth engine on
After having identified a big enough problem and created a product people love, the third phase is about attracting more customers and building virality into your product. Further it's about financing the growth by attracting external investors.