Question

What is meant by saying that a hypothesis must be testable

Answer 1

http://www.batesville.k12.in.us/Physics/PhyNet/AboutScience/Hypotheses.html

Answer 2

You need to have empirical (observable) evidence that can be mesaured by others (replicated). Many internal body changes could not be measured, so hypotheses about them were not testable. Now that we have methods to detect these body changes, they are testable.

I hope this helps a little more. Thanks for asking.