Please Explain Gauss elimination Matlab Code

Hi I am new to Matlab below is the code provided by my university and I am finding difficulties in understanding the code at the highlighted areas I am well aware of Gauss elimination process and I am good at math manually but I am facing lot of problem in understanding the code for Gauss elimination I got some basic knowledge on matlab of how to save function and execute the program successfully

Can anyone explain the code from line no 5 I got stuck up in understanding the logic in the program sequence

Thanks in advance

function [x]=gauss1(A,b)

[n,m] = size(A);
if n~=m; error('A is not a square matrix'); else

      % Forward elimination
      for k = 1:n-1,
          for i = k+1:n,
              if A(k,k)==0, error('Null diagonal element'); end
              r = A(i,k)/A(k,k);
              b(i) = b(i)-r*b(k);
              for j = k+1:n,
                  A(i,j) = A(i,j)-r*A(k,j);
              end
          end
      end