A bitmap is the term for how computers see a picture. It's a grid of squares of various colors, used like a mosaic to form an image. Just open up any picture in an editing program, and zoom it past 4x size, and you'll see the squares. But just that increase enough can matter with games, since again a lot of pictures are handled at once. Having all the pictures be 24 bit, means only a third as many can fit as 8 bit pictures. Those will be covered in their own pages.