Equalizers equations with GUI

function C1_var_Callback(hObject, eventdata, handles)
% hObject    handle to C1_var (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'Value') returns position of slider
%        get(hObject,'Min') and get(hObject,'Max') to determine range of slider
global C;
C(1)=get(hObject,'value');
set(handles.C1_val,'string',num2str(C(1)));

function plot_H_Callback(hObject, eventdata, handles)
% hObject    handle to plot_H (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
global C Fs;
equalizer_plot();

function equalizer_plot()
global C Fs;
[a,b]=coef();
H=0;
for i=1:5
    H=H+10^(C(i)/20)*abs(freqz(b{i},a{i},1024));
end
plot(1e-3*Fs*[0:1023]/2048,20*log10(H));
xlabel('Frequency [kHz]');
ylabel('Magnitude [dB]');
title('Audio equalizer feature');
axis([0 1e-3*Fs/2 -21 21]);
grid on;

function [a,b]=coef()
global Fs;
%Filtrul 1
Rp1=0.5;
Rs1=30;
Fp1=4.1e3/(Fs/2);                     
Fs1=4.5e3/(Fs/2);                     
n1=cheb1ord(Fp1,Fs1,Rp1,Rs1);
[b1,a1]=cheby1(n1,Rp1,Fp1,'low');
%Filtrul 2
Rp2=0.5;
Rs2=30;
Fp2=1e3*[4.25,8.75]/(Fs/2);                   
Fs2=1e3*[3.9,9.35]/(Fs/2);
n2=cheb1ord(Fp2,Fs2,Rp2,Rs2);
[b2,a2]=cheby1(n2,Rp2,Fp2);

Can anyone explain what is the purpose and what the equation refers to in such code

it's an digital audio equalizer