What is the Myth of Er?

This is a story told by Plato... but the Word "myth" used here actually meant Word or account, so there is a possibility that it could be an accurate account of what the man, Er, reported or dreamed, similar to the near-death experience/dream that Dante's works are based on.

In the story, Er lays on the battlefield for ten days before his body is collected, and tells a story about a near-death, or post-death, experience, which is a way that Plato instructs his students about the immortality of the soul.