Three-act structure
Also said as Inciting incident, Setup, confrontation, resolution
A story in three parts: set up a normal life until an inciting incident breaks it, struggle with the consequences, then resolve them. Syd Field made the shape standard in screenwriting.
It gives you a schedule for what the player should feel, and a way to spot a game with a good first ten hours and nothing to resolve. The inciting incident is the most useful piece: if nothing has broken yet, the player has no reason to care.
Say it
“The inciting incident arrives too late. Keep the first act to 20 minutes: the village is attacked at the end of the first quest, and the player cannot go home after that.”
Where you’ve seen it
Portal is close to a clean version: test chambers as the setup, your escape from the fire at chamber 19 as the turn, and the fight with GLaDOS as the end. In Half-Life, the resonance cascade is the inciting incident.
Not to be confused with hero's journey
Three acts is a schedule for any story. The hero's journey is one particular plot, with particular events in it, that can fit on that schedule.