That, in a nutshell, are the Big 6: items that are important for your character to continue working well mechanically as the challenges ramp up. They're not all equally important to every character -- for example, many arcane casters bow out of the AC game after the first few levels in favor of other types of defense against physical attack such as Mirror Image, Invisibility, and so on.
There's no hard and fast rule as to when you get or upgrade these; but you can get a rough idea of where the game developers expected people to get them by looking at the automatic bonus progression from Pathfinder Unchained, which replaces the Big 6 with built-in bonuses.