Playing around and saw the box "show hidden items (1 at this break point)"
What does it mean and why would I place it there.

This means that under the breakpoint you are in there is one hidden item. If you check that box it will show the hidden item and you can edit the settings or show it again using the Display settings.

The purpose of Hide check box and how to stack Row and Columns on each other
http://youtu.be/9OQJJ6M2Euw