What does recipe in a nut shell mean?

"In a nutshell" is a commonly used phrase that translates to "summed up succinctly or briefly into its key highlights," and is derived from its association with nuts stored with hard and protective external shells which hide their "meaty" interiors."

When one talks about "the recipe in a nutshell," one implies the "summarisation in essential components of any recipe's key ingredients (usually the minimum requirements) as well as its procedure" so that an interested but pressed-for-time viewer, reader or listener grasps the core of the subject without the risk of missing the essentials.

To clarify and simplify: It's akin to having a condensed representation of the recipe that focuses on what needs to be known or completed without omitting valuable insights into its making