Read the source of the allusion.

The ancient Roman scholar Pliny the Elder relayed an unbelievable story in which The Iliad (a poem hundreds of pages long) was written on parchment paper and inserted into the shell of a walnut. William Shakespeare later referenced his story when his character Hamlet uses the phrase "in a nutshell" to describe something that is compact

What is the meaning of the allusion in the sentence below?

So, in a nutshell, that's the story of my month long trip to Iceland. (1 point)

In conclusion

In a concise way

The meaning of the allusion in the sentence is that the story being described is being summarized or presented in a concise manner, similar to how the Iliad was supposedly condensed and stored in a nutshell.