For the people who don't speak the academic dialect:

DOI stands for Digital Object Identifier. It is a unique ID number that all mainstream academic articles receive upon publication.

The main function of the DOI is for copy-pasting into Sci-Hub to access the PDF of the paper you wish to read.

You can think of it as the equivalent of an ISBN, but for journal articles instead of books.

The DOI number of the paper you're looking at. You just need to set $doi to a string that contains the DOI. (In bash this would literally be doi=<some doi string>

A DOI is a bit different than a standard identifier such as a ISBN in that there are multiple 'registries'[0], and they need to be paid for (it is as good as it gets for persistence aspirations these days)

[0]https://www.doi.org/RA_Coverage.html

A variable representing the DOI of the resource in question, fill it in with whatever you're searching for (the DOI of a paper, for instance).

the DOI number identifying the article.