Do you like nuts? I do! There are dozens of them, from the popular almonds to the exotic macadamia nuts. You can have them raw, salted, sugar-coated, fried... And the best thing about them is they're packed with vitamins, minerals and healthy fat.
Today's idiom is a little tribute to nuts. The nutshell is the outter and harder part of the nut, which you don't usually eat. When you crack a nut, what can be put into the remaining shells? Not much, since most nuts are tiny. Then, if you hear someone say 'in a nutshell', expect just a few words afterwards. This expression is used to rephrase things simply and concisely at the end of an essay or speech. It's an equivalent of the better-known 'to sum up' or 'briefly' which you've probably studied at school, e.g.: 'in a nutshell, the film wasn't as good as the book'.
The idiom can also be used to present a topic in a straightforward, easy way, and it usually appears in titles or headings, e.g.: 'Benefits of swimming in a nutshell'.