According to plato, the sun outside the cave is

"Good"

Explanation: According to the allegory of Plato about the cave describes about the philosophy of analogy.In this theory he states sun as the metaphor which describes the reality's and knowledge's characteristics.

The cave indicates about the physical reality in superficial manner. In this analogy ,Socrates refers the Sun with the "good ".Thus ,it means that outside the superficial reality there is the actual reality of nature that is considered as good characteristic.