How to evaluate a postdoc offer, if I'm not sure I'd be able to do the work?
A postdoctoral position is first and foremost advanced training that prepares you to be an independent researcher above and beyond the training you received as a graduate student.  Your postdoctoral training should incorporate material that you are unfamiliar with, and a training plan for you to learn that material.  If you are concerned that you barely understand the research project, I would recommend identifying specific aspects of the project you are unfamiliar with (background literature, methodology, unanswered questions, etc) and talk with the PI about how, if you accepted the position, the lab would set up a specific training plan for you to learn that material.  I would only decline the position if they seem unwilling or unable to provide that training.

Personally, I struggled with the same concern when I started my post-doc, but my postdoctoral PI repeatedly reminded me that she hired me not because of what I already knew, but because she knew I was excited about the field and was confident I could learn. I've been productive thus far, and feel well positioned for the coming job search.  Plus, now I have distinct skill sets from my predoctoral and postdoctoral work, which helps distinguish me from others in the field.