Have you seen a nutshell? Think of how small it is and how little it can hold. So, in a nutshell means in summary, or in as few words as possible. Often used in situations where you could use 'in summary' instead.
For example:
▪️ This book is about successful businesspeople and how they reached the top. In a nutshell, it's about how to grow a successful business.
▪️ I don't want the long version - just tell me what your idea is in a nutshell.
▪️ In a nutshell, I think we should proceed cautiously and make sure we develop a robust backup plan in parallel.