What is the red circular diagram on the upper left? I watched a physics lecture that I understood nothing of in which the lecturer drew a similar diagram on the blackboard.
Is the Poincaré disk, a compact hyperbolic 2-d manifold. A space with negative curvature and maximal symmetry, the equivalent of a sphere but with negative curvature if you will.
You may have seen something similar in holography and AdS/CFT context. Or in an Escher painting.
