Nature/the world is what is being described. QM, thermo, fluid dynamics, signal processing, information theory, probability, etc.. are descriptions which have been devised to depict and communicate nature/the world. We never say that da Vinci's painting of Mona Lisa is so good that the historic woman who was Mona Lisa was paint.
They're more than descriptions because (most of them) make testable predictions about the world. Doesn't mean they're correct, of course, but making predictions is really important.