The rules so far have detailed the resolution of individual actions and short-term fiction. Let's build it up from the ground:
Individual actions, as described in Action Rolls and Consequences represent one thing that a character does, sometimes with or against other characters. The action is the smallest building block of the game. That does not mean an action is over in seconds - it might be if it is a fight, but a legal argument in front of the local lord while also being resolved with an action roll might last for hours.
Rarely tracked explicitly, a combat round is the time it takes for each character to complete one action. Combat rounds do not have a fixed time in seconds, but if it is ever needed, assume that one combat round takes about 10 seconds, on average. The GM can rule specific combat rounds to be longer or shorter, as fits the fiction.
Remember from the chapter about Action Rolls that an action in combat is not one swing or one shot with a bow, but a series of exchanges, back and forth, block and dodge, swing and hit and miss; or a series of aiming, shooting, reloading, etc.
With the action resolution system of Fallen Empire, an entire combat rarely lasts longer then a few rounds, if that.
While an action describes one thing that one character does, a scene is the collection of what all of the characters are doing in that moment. Scenes are a basic concept of storytelling and appear in movies, theatre plays and other forms of fiction.
The effects of spells and many other temporary advantages and disadvantages typically last for the duration of one scene.
As scenes flow into each other and form a dramatic arc with a beginning, middle and end, they come together into an adventure. An adventure is everything that happens during a fairly continuous frame of time as part of the same story. An adventure would be the entire movie or play, or a book.
Downtime is the time between adventures, when the characters are not chasing evil, saving the world, protecting or plundering villages, etc.
Downtime is when the action is over, the characters have a break or spend a considerable length of time less eventful than during an adventure. Downtime can last for a couple of days, weeks, months or even years, though days and weeks is probably the most common.
During downtime, characters engage in Downtime Activities.
Every adventure should be followed by Downtime, though the GM can make exceptions and press the party on if the fiction demands it.
The GM can also grant the characters Downtime, possibly with limited activities to choose from, during uneventful times such as a long journey by ship. Travel on foot or horse does not usually grant downtime as the characters need to be active to travel.
A series of adventure and downtime cycles with a collective plot forms a campaign. Where an adventure is a movie or a book, a campaign is a series. The adventures within a campaign often have the same antagonist, or the same goal, with each individual adventure being about a part of the larger plot. This may not be obvious at the beginning of the campaign, and part of the campaign often consists of the party slowly understanding how everything is connected.
When at the end of your second campaign you find out that there is a larger evil behind the two antagonists that you have defeated in your two campaigns, and everything was part of a larger plan, one that spanned not just two campaigns but also two eras, you are part of a campaign arc - a long-term narrative that provides background and meaning and ties together seemingly unconnected pieces of the various adventures and campaigns.