This program lets you rotate a tesseract and a 3-sphere in four dimensions.
A tesseract is a four dimensional cube like object. Basically, a tesseract is formed from a cube in a same way as a cube is formed from a square or a square is formed from a line.
A 3-sphere is a four dimensional sphere like object. Its form is much more complex than the one of a tesseract and is much more difficult to understand (even I don't really understand it). The rendering of the 3-sphere is executed so that the program draws dots that are on the surface of the 3-sphere and as the dots move they leave a trail behind.
The program uses cross-eyed 3D rendering. Basically, if you can cross your eyes so that the two images next to each other combine so that the resulting image is sharp you will see the image in real 3D. The rendering is done with SDL and OpenGL.