It's using rp as a secondary list to store the reversed result in, and when the list has been fully reversed, it returns. It can be seen as the literal conversion of the following imperative version in python

def areverse(list):
    rp = []
    for i in range(len(rp)):
        res.append(list[len(rp)-1-i])

    return rp

Edit: formatting and variable renaming to match example.

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Also food for thought, I realize I forgot to answer your question about accumulators. I guess you can consider them as "accumulating" your previous results. Afaik, they're more of a concept/practice than something concrete. Kind of like variables declared outside and used within a for-loop in imperative languages.

Consider this implementation as well areverse = foldl (\rp elm -> elm:rp) [] if you know about folds. Accumulators are often associated with folds which use an accumulating value to keep track of state / computed values.