You don't have to read this to understand the story. You can read it if you want, but it's not required.
We know Princesses Celestia and Luna used the Elements of harmony to turn Discord to stone. We also know that Celestia used those same Elements to banish her own sister to the moon. But when Luna helped Celestia turn Discord to stone over a thousand years ago, before her own banishment, she did not have harmony with herself or her sister. A thousand years ago, there was nothing but hatred in her heart. Until someone changed that.