The author did r(1:n*m) to get r into a 1-dimensional row vector. If r had remained a 2-D matrix, then hist, like a lot of MATLAB functions, gives you a hist for every column separately. The author didn't want m histograms, they wanted only one histogram so they make r 1-D. However, this could have been done simpler, and in a more MATLAB-ish manner, by simply doing r(:) as their comment mentioned.
The 1:limit was to specify the centers, and hence the number, of the bins.
