What is a tetrahedron?

A tetrahedron is a complicated word for a relatively simple 3D shape. It is also known as the triangular pyramid. It is a pyramid where the base is a triangle—or, even more specifically, all four faces of the shape are triangles. Think about the pyramids of Egypt, which have four triangular faces and sit on a face shaped like square. Tetrahedrons are like that, but the base is a triangle, resulting in four total faces rather than five. To further define it, it has six straight edges and four corners in addition to the four triangular faces.
There can be different kinds of tetrahedrons based on the faces: scalene, isosceles and equilateral. There are fun geometrical things to be done with tetrahedrons, but that can wait for another day. Feel free to just google "tetrahedron" and you will get lots of pictures that will help you visualize them.