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 need to test by passing something more complex to the function besides ABC. songDecoder("AWUBWUBWUBBWUBWUBWUBC"); This will help you understand why you're getting errors.

can't say for sure. might have something to do with this- "when using a regexp you have to set the global ("g") flag;"

I had the same problem at first. You've got to test using a version that has consecutive WUBs. I suggest testing this by passing "AWUBWUBWUBBWUBWUBWUBC" to the function so that you can see where the additional spaces are coming from and how to remove them.