Subject: What is 1,3-Hexadiene, and how is it relevant?

I have an example in my book, which looks like this:

CH2=CH-C=CH-CH2-CH3
       ^
  CH3-CH-CH3
(The ^ is where the shorter branch is attached to the longer one)

Its askes to name the compound- the solution in the book is 
3-isopropal-1,3-hexadiene.

I understand the "3-isopropal" part...but where did they get the 1,3-hexadiene? 
I know the prefix is hexane cause it has 6 C atoms...but the rest??

Re: What is 1,3-Hexadiene, and how is it relevant?