d == 1 where d is a vector, produces a logical vector as output -- a vector of true and false values.
When you use a logical vector as a subscript, that is called "logical indexing", and it selects for output all locations where the vector is true. In this context, those are used to select rows -- so the m(d==1,2) selects column 2 of all rows of m corresponding to the positions where d==1 is true.
v = unique(m(d==1,2))
acts the same as
temp = find(d==1);
v = unique(m(temp, 2))
