The hard, protective covering of a nut is called a nutshell. If you hear someone say "in a nutshell," they are most likely summing something up in a few words.
It's more common to refer to a nutshell as simply a shell. Most people use the word nutshell figuratively. When you say, "in a nutshell," you mean "to sum it up," or "to condense a large amount of information into one short sentence." For example you might say, "In a nutshell, The Odyssey is the story of a guy who went on a long boat trip."