matrix region sum function

matrix region sum function

I'm having trouble understanding this piece of code. This function
determines the sum of a selected area in a matrix. You are given matrix
and coordinates of the top left(A) and bottom right(D) corner of the
rectangle.
def matrixRegionSum(matrix, A, D):
if len(matrix) == 0:
return
totalSum = 0
for i in range(A[0], D[0] + 1):
for j in range (A[1], D[1] + 1):
totalSum += matrix[i][j]
return totalSum
The part I am stuck on is inside the range() function. I'm not sure how
A[0], B[0], A[1], and B[1] access the matrix. Could someone explain? I see
A[0] as the first value in an array, but a matrix is 2D. Could someone
explain to me the logic behind this function?
Thank you for the help! much appreciated!