When you look at the full size image, you can imagine that it is made up of a whole lot of little 800 x 600 pieces. Think of it like a picture puzzle, after you have it all together it makes a nice big picture. The puzzle is made up of a whole lot of small pieces.
Now if you take one of those little pieces out and look at it on it's own, that is the same thing you are doing with a 100% crop.
If you look at the image at the left, which is our picture puzzle, you will see all the pieces and each piece is 800 x 600. Now if you were to pick up one piece you would see just a small part of the large image.
So this is an 800 x 600, 100% crop.