Plot the magnitude of coefficients against frequencies of DCT transform
Actuually, that code doesn't work without defining axis t variable, and frequency f, regarding to what that code does, is simple:

x=sin(2*pi*f*t);        %x is a sinusoidal vector of frequency f and length t;
dct_transform=dct(x);   %dct_transform is a vector containing the unitary discrete transform of x
figure(1), plot(dct_transform); %plots the the unitary discrete transform of the sinusoid against default axis in Matlab


So you are just plotting the discrete transform vector against sample position, wish is what Matlab does by default.