d is an array of 10 element
r is an array of two element
d(1:5:6)=r says set the values of some elements in d with the values from r. Set the value of d[1], then d[1+5] until the index reaches beyond 6.
if you set it like d(1:2:6), then it will not work because d(1:2:6) will have three elements (d[1], d[3] and d[5]) while r has two. For the assignment the both sides must have the same number of elements.