"Product/market fit means being in a good market with a product that can satisfy that market."
- Marc Andreesen
In the tech industry you don't have a business until you have a product. In particular, a product that a lot of people want.
You've achieved product-market fit when you have a product that the market wants and you can tell because the market is pulling the product. That is customers are buying the product as fast as you can make it.
It is too early to focus on building the business until you have product-market fit.