Bayside Beat Tutorial - overflow

In the Bayside Beat Dreamweaver tutorial I get to 'part 7' and find that the menu on the site I have diligently worked through doesn't work.

I compare my code with the code supplied and find that the authors have added a line:

header {overflow: hidden}

They don't reference this in any of the tutorial and I note that it does not appear in the sample code for part 6, so has been introduced in part 7.

The additional code prevents the contents of the header from overflowing the header box.  If yuou change the value for the overflow property to 'scroll' scroll bars appear around the header section. If you turn it to auto it has the same effect as 'hidden', i.e. no scroll bars.

As I am learning I wanted to try to reduce the content that was causing the overflow in the first place but it doesn't seem as though the content is actually overflowing?

Could anybody point me in the right direction as this is my first foray into Dreamweaver and I would like to understand what is at the root of the issue rather than simply applying a patch to it.

Thanks

Phil