Is figure 3 a frequency response?

Hi, I have the following code[attached png]. I am practising to write a program that will create a filter for an audio signal. First I am running some exercises that will teach some basic concepts before attempting the task. Exercise is attached in png file. The exercise is supposed to introduce how to perform FFT on MATLAB. I don't understand the section of code starting at line 15-ending at 18. What is this supposed to do? Question: is the figure 3 called a frequency response?

[dpb]

Well, yes, it is computing/displaying the one-sided PSD. Line 15 is simply computing the frequency vector corresponding to the sampling parameters of rate and number of samples.

What is your reference text? Surely it has the basic relationships defined therein???