It’s a so-called mask. The thing is, you get the RGB value all in one integer, with one byte for each component. Something like 0xAARRGGBB (alpha, red, green, blue). By performing a bitwise-and with 0xFF, you keep just the last part, which is blue. For other channels, you’d use:
int alpha = (rgb >>> 24) & 0xFF;
int red = (rgb >>> 16) & 0xFF;
int green = (rgb >>> 8) & 0xFF;
int blue = (rgb >>> 0) & 0xFF;
In the alpha case, you can skip & 0xFF, because it doesn’t do anything; same for shifting by 0 in the blue case.