It's constructing anonymous functions. See http://www.mathworks.com/help/techdoc/matlab_prog/f4-70115.html and http://www.mathworks.com/products/matlab/demos.html?file=/products/demos/shipping/matlab/anondemo.html.
You could achieve the same result more simply by computing the values without building the function, like this:
f = 0.5; % Set f to whatever it's supposed to be here
T = linspace(0,1,1000);
Y1 = sin(2*pi*f*T);
Y2 = 1/2 * sin(6*pi*f*T);
Y = Y1 + Y2;
plot(T,Y1,T,Y2,T,Y);
legend({'S1(t)','S2(t)','S(t)'});
The advantage of making the functions is that they are available for use later, but if you don't need that you might find it easier to work without them, using code like the example above.
