OK, what it's doing is looping through each row, from row 1 to the row above the formula and checking if columns G, D and P of each row match the same columns in the current row. When it multiplies the results together Excel will convert the TRUE/FALSE values from these checks to 1 and 0 respectively, so the multiplications will result in a result of 1 (if all three columns match) or 0 (if one or more the columns don't match).

So, in your example file, the 6th row would produce an array like this {0,0,0,0,1}

MAX then picks the largest value from this array. If there have been no matches on any of the previous rows the largest value in the array will be 0, otherwise it will 1.