bi-level co-occurence image 1

i have a image and i like to calculate the bi-level co-occurence (a coocurence count is the number of pixels occurs at a fixed distance and orientation). i'm used the 'graycomatrix' function of matlab and i dont interstand why can varie the values of distance d (1,2,4,8) and the orientation(vertical, horizontal, major diagonal and minor diagonal) please help me