What does this k mean?

[MathsionYang]

// Every grid offset
for(int k = 0; k < 4; k++)
{
computeOffset(k, off_x, off_y);
for(int i = 0; i < m.size(); i++)
{
// Keypoint indices
kIdx_1 = m[i].queryIdx;
kIdx_2 = m[i].trainIdx;

			// Source and destination cells
			gIdx_1 = getGridIdxFromPoint(kp_1[kIdx_1].pt, off_x, off_y, _im1->cols / N, _im1->rows / N);
			gIdx_2 = getGridIdxFromPoint(kp_2[kIdx_2].pt, off_x, off_y, _im2->cols / N, _im2->rows / N);

			// Instantiate cellMatch object and add to grid
			cell_matches[k][gIdx_1].push_back({gIdx_1, gIdx_2, kp_1[kIdx_1], kp_2[kIdx_2], m[i]});

			// Increment match count
			cell_bins[k][gIdx_1][gIdx_2]++;
	}
}