Focus Peaking—rookie question.

I'm extremely happy that you have created this product, it seems like it will be a lifesaver. My eyes get pretty bleary by the end of the day, but I haven't used focus peaking functions before and I'm hoping you can help me understand it better. 
When I turn on Focus Peaking the image shows areas covered by tiny green dots, a few solid lines, and areas without any overlay. How can I use that tool to tell where the image is truly sharp and how sharp it is? I shoot a lot of products and closeups of hands so my depth of field is unfortunately usually pretty shallow. 
And again, to determine point of maximu, sharpness, which preview do I use? Red or Green?
Thank you for your help...