Convolution of two signals

Hello, can anyone explain me this part of the code, i don't understand very well when starts the for

m=length(x);
n=length(h);
X=[x,zeros(1,n)]; 
H=[h,zeros(1,m)]; 

    for i=1:n+m-1
    Y(i)=0;
        for j=1:m
            if(i-j+1>0)
               Y(i)=Y(i)+X(j)*H(i-j+1);
            else
            end
        end
    end