What Frank's (And Claire's) Asides Really Mean
Perhaps the biggest twist of House of Cards season 6 is the reveal that Frank Underwood's many asides to the audience were seemingly excerpts from a secret audio diary that he kept throughout the show. It's perhaps a little far-fetched that Frank would leave record a villain monologue of all his terrible acts (which, if found, could ruin him), but it's significant that he trusted Doug's loyalty so much that he would bequeath it to his right-hand man in death.
Of course, this raises the question of whether Claire breaking the fourth wall also means that she kept an audio diary, and that Doug's own asides to the camera were an indication that he kept a record of his thoughts as well. However, this twist probably isn't meant to be interpreted quite so literally. It stands to reason that Frank would use an audio diary to record his most private thoughts and musings, and he shared those same thoughts and musings with the audience.
What House of Cards Says About Power & America
While some might believe that House of Cards is about American government being under attack by an unprecedented level of deviousness, corruption, and evil, the lingering message of the show - and season 6 in particular - is that what Frank and Claire did isn't really out of the ordinary. There's no indication that America is spinning towards open fascism, since that wouldn't really be Claire's style. The show is really about a more insidious form of tyranny, which is more difficult to pull off, but also more difficult to expose and defeat.
In a particularly telling scene in the final episode, Mark Usher goes to visit Bill Shepherd at his countryside home. Though he started out the season as a formidable antagonist, Bill is left weakened by his deteriorating health and has become a sideshow feature on news programs - raving against Claire, and being taken seriously by no one. Mark tells Bill that his own sister, Annette, is preparing to oppose him and presumably take Claire's side in the public eye. Bill asks Mark where he thinks the country will be in ten years, perhaps imagining some kind of apocalyptic scenario, but Mark has a more measured response. "Things don't really change, Bill," he says. "They just fade away."
A later scene with Bill Shepherd continues this thought, as Bill is going through a series of paintings from various points in history and countries around the world - including Harry Brooker and William Hogarth. It's here that the show's title gets name-dropped, with Bill looking at Jean-Baptiste-Siméon Chardin's painting The House of Cards. Bill says that all of these paintings, "have one thing in common: children watching, waiting for the cards to fall." It's a summary of the show's attitude towards power as something fragile, that can be brought tumbling down by the smallest mistake. Frank Underwood saw his own downfall coming, but ultimately couldn't do anything to stop it, and it seems as though Claire's own reign is ultimately doomed to be fall as well, since keeping the house of cards together means constantly playing a near-impossible game.