The main idea is the message the author is trying to say. The main idea should provide the story's importance, and the lesson or moral that can be learned from reading the story.
I'm assuming you mean the Boston Tea Party Act? If so it was led by The Sons of Liberty (A rebel group led by Samuel Adams to protest against Parliament) to protest against Parliament's "Tea Act". It...
A stated main idea is when the author says the main idea right in the story where an implied main idea is where the author gives you clues to the main idea and you have to infer it.