Matt's answer of
is the best way, at least in current versions of MATLAB, because it is most clear what it is doing. But this only applies in 2017 releases and beyond.
Other solutions are less clear, but pretty simple. For example, this should be almost as obvious:
[i,j] = meshgrid(1:n);
result = i + j;
result = cumsum(ones(n),1) + cumsum(ones(n),2);
result = repmat(1:n,n,1) + repmat(1:n,n,1).';
Any of these should be quite clear, and all are equally valid. If unclear still, then it really is time to start reading the getting started tutorials in MATLAB. If you do not follow what they did, then look at each piece of the code. See what it does for some value of n. Read the documentation, perhaps for meshgrid, repmat, bsxfun, cumsum, etc., as appropriate. You won't learn MATLAB until you start reading the documentation.