Given an array of integers nums, answer queries of the form: what is the sum of the elements between index left and index right, inclusive? Summing the range directly answers one query correctly, but it re-walks the same ground every time. In a system where the array stays fixed and the same range gets queried over and over, that's wasted work — a prefix sumPrefix SumA precomputed running total, where prefix[i] holds the sum of every element before index i. Once built, the sum of any range can be found with a single subtraction instead of re-adding every element in that range. array, built once, turns every future range query into a single subtraction.