How gaining understanding of your environment freed you from the limited reality it presented you.
The initial usage of the story was to illustrate how political indoctrination could present a false reality to ardent followers "the shadows of cave fire for those raised in the cave", and how education could "open the eyes and show them the true light" (the fire in the sky/sun vs the fire in the cave). "...even if one was dragged out of the cave".
This was later extended to demonstrate that if humans learned about their own emotions, they could learn to reason decisions, rather than allow emotions to guide them exclusively.
Tangentially, this rough concept in early western philosophy evolved into the religion of the Sun, and had significant influence on early Christianity.