Definition:
A Value Hypothesis tests if a product is valuable to potential customers. There are successful startups who no longer have a valuable product. These companies die. A value hypothesis determines if a customer should adopt a product into their lives. A value hypothesis turns a value statement into a question.
A hypothesis is an assumption that can be validated through experimentation and observation. Similarly, a Value Hypothesis should be a testable statement that can be validated or refuted when facing your prospective partner.
Further Reading:
Book: The Lean Startup -by Eric Ries