Any one tell me why we write this 0xff000000 in filters?

Here is my code for GrayScale filter want to know about this

class GrayScale extends RGBImageFilter {
  @Override
    public int filterRGB(int x, int y, int rgb) {
    int a = rgb & 0xff000000;
    int r = (rgb >> 16) & 0xff;
    int g = (rgb >> 8) & 0xff;
    int b = rgb & 0xff;
    rgb = (r * 77 + g * 151 + b * 28) >> 8; 
    return a | (rgb << 16) | (rgb << 8) | rgb;
    }
}