Need help with filters and Karplus-Strong algorithm (guitar strings)

Hello everybody! Firstly, I am having some trouble to understand the filters used in the code below...I have to play a music note as described below. Secondly, I need to play a sequence of music notes separately and later all together.

Any help will be apreciated.

fs=44100; 
A=150; 
F=linspace(1/fs, 1000, 2^12);
x=zeros(fs*10, 1);
delay=round(fs/A);
b=firls(42, [0 1/delay 2/delay 1], [0 0 1 1]);
a=[1 zeros(1, delay) -0.5 -0.5];
[H,W]=freqz(b, a, F, fs);
zi=rand(max(length(b), length(a))-1,1);
note=filter(b, a, x, zi);
note=note-mean(note);
note=note/max(abs(note));
hplayer=audioplayer(note, fs);
play(hplayer);
hplayer;