Hypothesis testing can be a tricky subject to explain.
The book’s* explanation is quite good; you should re-read chapter 9.
I have created a very simplified
cartoon-style introduction of the subject.
See the tutorial.
*
Coolican, H. (2006). Introduction to Research Methods
in Psychology (3rd ed.). London: Hodder Arnold
ISBN 13: 978-0-340-90757-3