Taking a special image derivative using convolution

I have two images at different times. The first image is im1 at time t1 and the second image is im2 at time t2. I want to take the spatial derivative with respect to x. Could someone help me understand the following MATLAB code?

    fx = conv2(im1, 0.25 * [-1 1; -1 1]) + conv2(im2,  0.25 *[-1 1; -1 1]);

I would be thankful to if someone can shed light on this kernel. What type of image derivative the above mentioned code is?