MATLAB: WHAT DOES THIS MEAN

matrix

 d = d( : )

: is the colon operator. As per the documentation:

 A(:) is all the elements of A, regarded as a single column.

So, your line just reshapes d as a single column. It is equivalent to

d = reshape(d, [], 1);

fprintf('\nIteration number = %d \n', m);

This probably means that you are referring to something like

d(3,3)

while the variable d does not exist.

You might have cleared it somewhere before calling it, or it may not have existed at all.