The expression "in a nutshell" generally refers to a concise explanation of something. When speakers want to cram just the essential points into as few words as possible, they are making their point in a nutshell. Something that’s said in a nutshell is succinct and to the point. While a speaker may use only a few words or sentences, he includes every necessary piece of information when speaking of something in a nutshell.
As with all languages, English uses idiomatic expressions. These phrases are commonly understood expressions that convey an idea with usually colorful language or metaphors. New idioms are added to common usage from time to time, coined by events, new technology, or other social activities. Older idioms stick around, often long past the time when the events or ideas they reference are remembered. for instance, the idiom "it’s raining cats and dogs" is so oblique that the only way to understand that it simply means "it’s raining very hard" may be to have it explained.
If something has been explained in a nutshell, the subject has been explained with just a bare minimum of words. This expression has about it both a sense of brevity and of complexity. Topics that are explained in a nutshell might seem simple or small, but they generally can be expanded upon if the speaker chooses to elaborate on the ideas instead of using brevity to convey his message.
Trees are such an important part of the human world that there are many expressions in many languages that relate to their aspects and functions. The expression, “the apple doesn’t fall far from the tree” means that a child will grow up to be much like a parent. “Sticks and stones may break my bones” is a self-defense nearly every child has repeated. Someone who can’t see the woods for the trees is missing the big picture.