It's the "stem" part that covers the top of the fruit. It's color is purely ornamental.

The calyx color is distinctive to the variety. it can be helpful for seed savers in distinguishing seed purity vs. a possible cross pollination issue.

If you don't save seeds then it has no meaning other than ornamental as already mentioned.

Dave

Thanks !

In generalities most asian eggplants have purple calyx and European types green calyx.There has so much interbreeding that it really makes little difference except for appearance in the newer varieties.