Saturn's 'hexagon' is a large persistent cloud-pattern formation around the planet's 'North' pole.
It was discovered as a result of analysis of images from NASA's Voyager mission in 1981. And subsequently confirmed by the Cassini probe. [see photos courtesy NASA]
Various explanations for the pattern (none as yet generally accepted) have been suggested. They include the effects of turbulence, convection, and sheet-forming vortexes, or some combination of the three - see Wikipedia.