Need help in understanding the following FFT code (line %1 to %4)

function [ y ] = FFT_DIT_R2( x )

p=nextpow2(length(x));

x=[x zeros(1,(2^p)-length(x))];

N=length(x);

S=log2(N);

Half=1;

x=bitrevorder(x);

for stage=1:S;

for index=0:(2^stage):(N-1);

for n=0:(Half-1);

pos=n+index+1;

pow=(2^(S-stage))*n; %1

w=exp((-1i)*(2*pi)*pow/N); % 2

a=x(pos)+x(pos+Half).*w; % 3

b=x(pos)-x(pos+Half).*w; % 4

x(pos)=a;

x(pos+Half)=b;

end;

end;

Half=2*Half;

end;

y=x;

end