cellfun is just a convenient way to iterate over all the elements of a cell array. It pass each element in turn to the function it is given. In your particular case, the function is an anonymous function with one input (called x, the name doesn't matter) which returns the sum of x.IsBranch. If we were to deconstruct that cellfun line, it would be equivalent to:
mdlDefaultNumSplits = zeros(size(MdlDefault.Trained)); %cellfun automatically create the output array
for iter = 1:numel(mdlDefaultNumSplits) %cellfun iterates over each element
x = MdlDefault.Trained{iter}; %and dereference the cell content
%then it calls the function which in this case does
z = sum(x.IsBranch);
%and put the output of the anonymous function in the output matrix:
mdlDefaultNumSplits(iter) = z;
end
So x is the content of a single cell of MdlDefault.Trained which according to your first screenshot is a CompactRegressionTree. I don't have the stats toolbox so can check what IsBranch is. Since it's not listed as a property, I assume it's a function of CompactRegressionTree. Probably one that returns a logical array.
