Understanding code

Hi All,

Okay so I am a C programmer who is now programming in MATLAB. For our first college asignment I have to plot 3 sinusoid graphs of 4000 Hz with 25 samples per period. I also have to start from -T. The thing is I have the code. The whole class has the code i just want to understand it better.
Things I don't understand in the code:
* what is the point of fs
*i dont get how 3 graphs result from this

the x1-x3 functions are defined by our teacher.

Thank you:

code:

f = 22050; %frequency of sinusoids
T = 1/f; %period of sinusoids
fs = 20/T; %at least 20 samples per period
tt = -T:(1/fs):2*T; %3 cycles starts at a negative time equal to -T
x1 = 24*cos(2*pi*f*tt+8.8);
x2 = 20*cos(2*pi*f*tt+2);
x3 = x1+x2;

subplot(311);
plot(tt, x1), grid on;
title('x1');
axis tight
subplot(312);
plot(tt, x2), grid on;
title('x2');
axis tight
subplot(313);
plot(tt, x3), grid on;
title('x3');
xlabel('Time (sec)');
axis tight
orient tall