The snippet that you want to focus on is part of a switch statement. The variable nargin is a integer that tells you how many arguments are passed to the function, and varargin is a cell array that holds the arguments passed to the function. Similarly, varargout is a cell array that holds multiple outputs.
The switch statement has an argument of nargin (the number of arguments provided to the function. The line 'case 1' handles the case for which nargin=1. The next line bs=varargin{1} takes the first argument passed to the function (in this case the only one, since nargin=1) and stores it in bs. The next few lines defines a matrix, called boundary. Finally, the line 'varargout{1} = boundary(:,bs);' takes specific columns of boundary, and stores them in varargout.
