It is probably a stupid question. I am busy trying to teach myself this game. I have encountered the statment that you can only benefit per one First Aid check per Act in an encounter. I, however, have no idea what an Act is...

Ah you have encountered perhaps the two highest "threat" rated issues in the game for being a bit vague - though neither fatal to "whatever feels right at your table".

The hardbacks are supposed to cure that a bit.

An Act is "one encounter" considering an encounter being what goes on between "commercial breaks" (imagining the action as a TV show) or other "change ups in situation". That proviso is the main difference from just saying "one set of actions/fight". You're exploring the graveyard in "Act One", then the dead start to rise "Act Two, zombie fight", you might still be fighting zombies in same place when "Act Three, the necromancer shows himself" starts as a Necromancer and his pet uber zombie appear. The Act structure ensures PC's get the mini-respite of the Rally Step between those events and also rations any "once per act" things.

In a mixed social-combat scenario, Act One might be "enjoying events at the fair/Scahffenfest" (a few free form encountners), Act Two might be "chasing down the escaped monkey for the menagerie owner", Act Three might be "fighting the thugs who want the monkey". The first two Acts may stretch over different locations and deal with different NPC's but are both "held together by a single concept". The actual "game world time" an Act encompasses is flexible - it could be seconds or days.

The rules then suggest that 3 Acts make a good structure for an "episode" which is a larger "unit of narrative/story/plot" such as "The Fight at the Graveyard" or "Monkey Shines at the Schaffenfest". A basic "set up/intro"/"rising action"/"climax/bridge to next episode" structure.

Rob

Although your detailed description was pretty accurate, I would suggest avoiding the use of the term "encounter" when discussing an Act. An Encounter could be part of an Act, but an Act is a bigger story element than a single Encounter (in game terms).

For example, the Act might indeed be the Exploration of the graveyard to find the vampire's lair. Inside the Act, there is the scene where the PCs find their way to the graveyard itself. Then, they have a combat Encounter with guardian zombies. Finally, they find the mausoleum that houses the secret passage down to the tunnels where the vampire lives. Which then leads to the next (and most likely final) Act, where the PC find and face the vampire in his lair and (hopefully) defeat the evil.

If you're the GM, I'd take a look at the layout and wording of some pre-printed adventures, such as An Eye for an Eye. Notice where it sections the adventure into Acts.