Can someone please help me with this code?

Hi, I hope it's not to much to ask, if it is - my apologies.

I have this code I'm trying to understand how it works. It's used for analyzing pictures (by comparing them to one another).

My first question is regarding the sum command; what does it do to the pictures?

Secondly, what does the fspecial command defines in this code?

clearvars -except Struct
program =2;
addpath(genpath('G:\DATA SUM\ISI\310815\Aug.31,2015 15.44.55\TimtomlineData'));

if program ==2
    clear all
    close all
    clc
load('Cycle_1.mat'); 
Struct(1).file=Out_mat; 
load('Cycle_2.mat'); 
Struct(2).file=Out_mat; 
load('Cycle_3.mat'); 
Struct(3).file=Out_mat; %it goes on and on to 4,5,6,...

end 

%%Process script

[R, C]=size(Struct);
[R1, C1, F1]=size(Struct(1).file);
AddVol=zeros(R1,C1,F1);
AddFrame=zeros(R1,C1);

sum1=sum(sum(Struct(1).file));
sum1=squeeze(sum1)';
sum1(1,1)=sum1(1,2);
figure,plot(sum1)
hold on

%%Post Processing

H = fspecial('average',[10 10]) ;

for i=1:C
    AddVol=imadd(AddVol,(double(Struct(i).file./C)));
end

AddVol(:,:,1:8)=[];
[Row1, Col1, F3]=size(AddVol);
threesecframes=8*3-1;

% Avg data before stimuli

for j=1:threesecframes-1
    AddFrame=imadd(AddFrame,(AddVol(:,:,j)./threesecframes));
end

K1=10000;
K2=1;

% Frame Division

for k=1:F3
    DivideIM(:,:,k)=(imdivide(double(AddVol(:,:,k)),double(AddFrame)).*K1+K2);
end

% Filtering

for q=1:F3
    FiltImdat(:,:,q)= imfilter(DivideIM(:,:,q),H,'circular' );
end

%%Division of Data

maxLev2=max(DivideIM(:));
minLev2=min(DivideIM(:));

figure,imshow3D(DivideIM,[minLev2 maxLev2]), colormap jet

%%Filtering Data

maxLev3=max(FiltImdat(:));

minLev3=min(FiltImdat(:));

figure,imshow3D(FiltImdat,[minLev3 maxLev3]), colormap jet

%%Maximum Intensity Projection 

MaxIntensityProj=FiltImdat(:,:,26:33);

[R1,C1,L1]=size(MaxIntensityProj);

for f1=1:R1
    for   f2=1:C1
        MaxPrj(f1,f2)= max(MaxIntensityProj(f1,f2,:));
    end
end

figure,imagesc(MaxPrj),colormap jet, title('Max Intensity Projection')

%  to get the blood vessel patterns:
%  figure,imshow3D(Struct(3).file)

Thank you for your help,

Sigal.