It’s a measure of whether folks in black areas in swing states have the energy and will to fight voter rolls purges, poll closures, spurious ID requirements, long lines and all the other voter suppression tactics that the so-called Supreme Court has been allowing since gutting the VRA.
A reminder that in Florida in 2000, Republicans were experimenting with these disenfranchisement tactics (roll purges, closing polling stations, convenient road blockages in cities, the works). They won that state* by a few hundred votes in key districts, and these dirty tactics became a mainstay for the party. And that was before the SCOTUS basically said "racism is over, these voter protection laws are unnecessary now". Every vote counts, especially where they've made it difficult to do so.
* Gore probably would have won Florida, and thus the presidency, if the SCOTUS didn't halt the recount - in large part due to Roger Stone, one of the biggest pushers of dirty tactics. There are a small number of people who have caused a great deal of harm in this country.
if you're asking what mechanism could be used to quantify "voter enthusiasm", a poll of "do you intend to vote in this election" seems like a pretty easy and obvious answer
In Trump's case it is usually to do with his own inflated claims of rally attendance rather than any firm polling data. Donny really knows how to inflame his base, but his party's consistent underperformance in by-year elections suggests that doesn't translate into general election success.
Do you know what enthusiasm means? The article you linked has a number of examples of what data might lead someone to believe in the increased enthusiasm. Did you read it?