Equalizer play() function with GUI

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

global stop file_name C;
stop=1;
equalizer_play();

function equalizer_play()
global stop file_name C;
[x,Fs]= audioread(file_name);
[a,b]=coef();
l_piece=2*Fs;
Nb=round(length(x)/l_piece);
y=0;
 
for i=1:floor(Nb)
    piece=x((i-1)*l_piece+1:i*l_piece);
    for k=1:5
        y=y+filter(10^(C(k)/20)*b{k},a{k},piece);
        if(stop==0)
            break;
        end     
    end
    p = audioplayer(y, Fs);
playblocking(p);
    y=0;
    if(stop==0)
        break;
    end   
end

I want to understand what are the functions and the equations refer to in such code

it's an audio digital equalizer