can anyone explain the meaning of the line mov_gray(i​).image=mo​v(i).cdata​"

mov=aviread(pic_path);

n=100; s=1; e=n;

 for i=s:e  
   mov_gray(i).image=mov(i).cdata;       % mov_gray(i).image为第i帧图像
 end

Thanks in advance...