Act One of the Three Act Structure. It's the setup or premise.
It sets up the who, what, when, where of the story. Who is the protagonist? What is distinctive about them? Do we like them? Where is the story taking place? Toward the end of the act of the writer will introduce the main problem for the hero. It ends when we know what their plan is to set things aright.
In games, it's the tutorial where the player learn basics to play.
(Slay the Dragon: Writing Great Video Games by Robert Denton Bryant & Keith Giglio, 2015. Page 76-79, 82, 92-94)