This is a 4-dimensional polytope, so like a 3-dimensional polyhedron (tetrahedron, cube, octahedron etc.) but one dimension up. Just as an octahedron has 8 triangular faces, this has 24 octahedral "faces", hence the name. This is a distorted picture (because it's hard to draw things in 4 dimensions) - all of the edges are really the same length and thickness. Also, here's a video.