The food piramid is a diagram of foods in order of how much you should have. The bottom is the biggest because you should have the more of that, than of the upper levels.
Going bottom to top, the bottom has only one section, its for the grains. One level up hast two sections, the fruits and vegetables.
The next level has two sections too, the meats and milks.
To see one version of the food piramid, check out:
This site