A nutshell is the shell of a nut ;-), but...
"... in a nutshell" is an English expression. It means "this is the essential information (condensed) about x". It's meant to be sort of a fun phrase--not too serious.
You can compare it to:
- a summary
- "the SHORT version"
- "just tell what I need to know, nothing more"
Normally, in conversation, if you can reduce the essence of an idea to ONE sentence, you would then say "that's it, in a nutshell". Obviously the Oreilly books take more than one sentence to explain Perl, or Linux, but the intention is to deliver a quick, but complete explanation of the subject.
Samansk -- obviously English is not your first language. What country are you from? Are you working in an English-speaking country now?