Digital Fm demodulation

Hello, i have downloaded a code from matlab file exchange but i do not understand how it works, can someone explain it to me? It is used for demodulation FM. thanks

function x = FMdemod(y,beta)

[r, c] = size;
if r*c == 0
y = [];
return;
end;
if (r == 1)
y = y);
len = c;
else
len = r;
end;

Fc = 40000;
Fs = 8000*20;
pi2 = 2*pi;

[num, den] = butter(5, Fc * 2 / Fs);

sen = 2*pi*beta*100;

%pre-process the filter.
if abs(den(1)) < eps
error('First denominator filter coefficient must be non-zero.');
else
num = num/den(1);
if (length(den) > 1)
den = - den(2:length(den)) / den(1);
else
den = 0;
end;
num = num)';
den = den)';
end;
len_den = length(den);
len_num = length(num);

x = y;
y = 2 * y;

ini_phase = pi/2;
for ii = 1 : size(y, 2)
z1 = zeros(length(den), 1);
s1 = zeros(len_num, 1);
intgl = 0;

memo = 0;
for i = 1:size(y, 1)
%start with the zero-initial condition integer.
vco_out = cos(pi2 * intgl+ini_phase);
if len_num > 1
s1 = [y(i, ii) * vco_out; s1(1:len_num-1)];
else
s1 = y(i, ii);
end
tmp = num * s1 + den * z1;
if len_den > 1
z1 = [tmp; z1(1:len_den-1)];
else
z1 = tmp;
end;
intgl = rem(((tmp*sen + Fc)/ Fs + intgl), 1);
x(i, ii) = tmp;
end;
end;
x = x;
x = decimate(x,20);