Hi Heather,
following Leila's example, if the critical path is 30 days and you have another (non-critical) path of 25 days, the project duration cannot be less than 30 days. This is what "which determines the shortest possible project duration" refers to. In order to complete all tasks, you need to have all the activities on the critical path completed, which will require you at least 30 days. Then, if any of the activities on the critical path gets delayed (for whatever reason), the whole project duration will increase, because activities on the critical path have zero float.
Hope this helps clarifying the point. I clearly remember Cornelius discussing the critical path in detail in the lessons dedicated to Project Schedule Management (Module 6), but I cannot point to the lesson number as I do not have access to the course anymore. You should be able to find a list of all modules and lessons, as well as a topic index when you access the PMPrepCast online.
Gabriella