This is an assumption that your product or service brings actual value towards the customers or users.
Another early hypothesis is The growth hypothesis.
You can use a Value Proposition Canvas to design and have a framework to create and iterate over these hypothesises.
Reference
Ries, Eric. The Lean Startup: How Today’s Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses. 1st ed, Crown Business, 2011.
Highlights or timestamps
The value hypothesis assumes that a product will deliver value to its customers, that is, that early adopters will find and embrace the product.
— ^5f35ae from The Lean Startup