Before the invention of writing, humans communicate with each other using the language that they know of. They usually tell stories by drawing on the dirt using a stick or draw on the caves to tell a story.
Humans that pass by those caves will understand the story, and some humans prefer to tell their children or the younger generation what happened, and the younger generation could pass it on to their kids.
Basically, they use art and passing it to a younger generation to preserve the stories before the discovery of writing.