The episode starts with Applejack going around town, seeing a variety of ponies with jobs that require little physical labor, such as investors and managers, looking tired. This makes her mad, she perceives them as being lazy, that they get tired doing work she doesn't perceive as difficult. She talks to Dash about it, and Dash talks to Twilight about it.
This makes Twilight mad, because she frequently overworks herself as a scholar. Then she thinks of 2 ponies who disprove Applejack's point even more than she. She invites Applejack in, and casts an Astral Projection Spell on her. She sends Applejack to Canterlot Castle, to secretly watch the Princesses for a day. Applejack sees firsthand how difficult their duties are, despite the fact that they don't harvest anything.
At the end, she learns that just because someone's job doesn't have a lot of physical labor does'nt mean it isn't hard.