A 3D projection of the 16-cell, one of the 6 regular polytopes in four dimensions.
4D Polytopes (or polychora) are the 4-dimensional analogous of the 2D polygons and 3D polyhedra.
Regular polychora are composed of a finite set of cells (polyhedra), all regular and alike, surrounding each edge in an identical way.
We cannot see a 4D polytope, but we can project it in 3D (in the same way as we make a flat drawing of a polyhedron).
The 16-cell is composed of 16 regular tetrahedra.
This is a particular central projection (perspective) of the polytope. Some cells intersect each other (but the edges don't), and the convex hull (wrapping polyhedron) is one of the 13 Catalan solids, the Triakistetrahedron.