A pyramid is a three-dimensional solid form made out of triangles. Some pyramids also use a square. There are two common kinds of pyramids: triangular pyramids that you make by fitting four triangles together, and square pyramids that you make by fitting four triangles together on a square base, like the one in the picture here. The pyramids of Egypt are square pyramids. You can also have pyramids with hexagonal or octagonal bases, or however many sides you like. If the base has an infinite number of sides, then it’s a circle, and the solid shape you’re making is not a pyramid but a cone.
To calculate the volume of a pyramid, first calculate the volume of the rectangular solid around the pyramid. That’s the area of the base multiplied by the height of the pyramid. Then divide that by three, because the pyramid doesn’t fill up the whole volume of the rectangular solid around it.