some questions in function convlove()

[zhangyingbit]

Hi, I have a question about the function convlove()
"val = (filter * pad_mat[j * strides[1]:j * strides[1] + filter_size[0],
k * strides[0]:k * strides[0] + filter_size[1]]).sum()"
why you multiply "j" with stride[i]? In the "for j in range(0, mat_size[0], strides[1]):" , there is a strides[1]. I'm confused about it. Thank you!