The relation back doctrine is a legal principle that allows an amendment to a pleading to be treated as if it had been filed on the date of the original pleading. This doctrine is significant because it ensures that amendments do not unfairly disadvantage a party by changing the timeline of events, thus maintaining fairness and consistency in the judicial process. It's crucial for allowing parties to correct mistakes or add claims without being barred by statutes of limitations, as it aligns the new allegations with the original claims.