An octahedron is a polyhedron (a 3D shape) with eight sides, which are all equilateral triangles, four of which meet at each corner. It has 12 edges and 6 corners. It is one of the 5 Platonic solids.
Simply put, this is two rectangular pyramids put together. A rectangular pyramid is a pyramid with the base of a square or rectangle. In other words, this is just a pyramid that, when flipped to stand on its point (like a spinning top) it has a rectangular from a birds eye view.