read more on this correct me if i'm wrong but the second argument (array.fill) is the initial value that the reduce method is being applied to

I don't see the code but to guess a bit -

[].reduce(f, acc)

acc is the initial value to use, for example when summing you start with 0:

const f = (a, b) => a + b
let acc = 0  // <-- second argument of reduce
for (const x of xs) {
    acc = f(acc, x)
}

Two big reasons why one might want to supply this is if it's empty (otherwise there can be no result) or if the accumulated value has a different type/shape from the things in the array so that the first value of the array cannot be used as the initial value (which is what happens when omitting acc)

i have a question/comment about this (sorry please humor a beginner)
i get that you're adding next to the lowest prev
i've just only seen reduce containing one function returning a single numeric value so this is my first exposure to a second argument being passed to the method
is the idea that by using return prev in the function followed by array fill, it's sort of doing a micro-level reduction and replaces an index in the fill array?

You should rename "prev" and "next" to "checkouts" and "nextCustomer" or something like that... This confuses the way that reduce() works.