Question

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 this 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, thats the story of my monthlong trip to iceland.

in conclusion
in a concise way

Explanation:

The allusion traces back to the story of the long Iliad condensed into a walnut shell, and Shakespeare's use of "in a nutshell" to mean compact. In the given sentence, the phrase is used to present a long trip's story in a shortened, compact form, matching "in a concise way".

Answer:

in a concise way