A thing explained In A Nutshell is explained in as few words as possible. Far into the past, important documents were carried around in walnut shells, which would then be bound and kept waterproof. So if you have something "in a nutshell," you've a shortened version that still covers every main point. Despite this, there are examples of long and celebrated works being written in handwriting that is so small the document would still fit inside the shell of a walnut.