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Code:

        if (FNR == NR)
        {
                arr[$0]++;
                next
        }

This goes through the first file adding each line into an array. It keeps a count of each lines occurrence. NR is the "record number," or the line number being processed since the default record delimiter is a new line. FNR is record number for this file. So for the first file, the two match. For the next file, NR keeps counting upwards but FNR is reset. The 'next' will skip to the next record (and all the code below), because we're done and the code below is for our second file.

Code:

        if ($0 in arr)
        {
                arr[$0]--;
                if (arr[$0] == 0)
                        delete arr[$0];
                next
        }

We're looking at our second file. We look to see if the line is matched with one in array (which holds all lines of first file). We decrease the count. If it's 0, we want to remove the "key" from the associative array, so that it's not looped through in the END statement's loop.

Code:

        {
                print $0 > "list2output.csv"
        }

Since that last block had a 'next' inside the if, this is only executed if the line was not in the first file. list2output.csv is exclusive to list2.

Code:

END {
        for (i in arr)
        {
                print i > "list1output.csv"
        }
}

What we have left in the array is put into list1output. What's left should be lines that appear in list1 more than they appear in list2.