“This is worth your time.”
The promise is made at the beginning of the story and has to be lived up to. The promise is always: “This story is worth your time.” To do that, the promise shows the reader what the story is about, or at least, that the story will be about something. That the story will lead somewhere good (in the sense of entertainment).
The promise is a tie in to the marketing you did beforehand (Premise, Title, Cover etc.). It needs to meet the expectations set up by the attention grabber and confirm that this is the story they are looking for. It does this through setting up the premise/situation and by conveying the mood of the content. Within the opening of your story you either hook a reader or lose them. Ideally you want an opening that makes the reader immediately forget that they are hesitant to start a new book/story. The objective of the promise is to make the viewer/reader forget their own lives and start transition into the world of the story.
Although stories can have very different structures, every story should follow the underlying principle of: set up, raising the stakes and conclusion. The promise is made within the set up of the story, right at the very beginning.
If you start your story by introducing a day in the life of a character, within that scene you are also making the promise to the reader. If within your Grabbing Attention phase you didn’t give the reader any information that let’s them know/assume that something is about to happen despite your seemingly slow opening, what you’re telling the reader is basically “this might be a story in which nothing happens”. So you want to make the promise first, not slowly build up to it.
What the promise contains
In stories the promise is always made up of two things: Conflict and Stakes. Conflict = something is about to happen, Stakes = this is why that something matters. In what shape or form those two things happen depends on the kind of story you are writing. Usually the stakes at this point are something universally relatable. In Finding Nemo the stakes are clear “Protect your young”. Although we don’t know any of the characters yet, we already understand why this is important to them. Whatever is important you your character, within the promise you are trying to show that by tapping into that aspect of it that is universally recognizable. We understand why someone would want a job that will have them work 24/7 if this is their IN to a career they have always dreamed about. We understand why someone who is an outcast would strive to gain people’s recognition, etc. Conflict on the other hand can take various forms. It can be obvious, immediate danger for instance, or it can also be a small conflict that is more recognizable from everyday life but that has the potential to escalate. It can also come in the form of mysteries. Mysteries are puzzles, a challenge to the reader. Here what the story conflict really is may not be obvious yet, but the challenge it will pose is. Conflict often also comes in the form of a problem the character is struggling with. A problem that is tied to their central weakness and which they have to solve.
As mentioned, while you have to make that promise, how you make it is completely up to you and the story you want to tell. Since the promise happens at the beginning it happens within your opening scene. Take a look at the opening to find some practicable ways of how to open your story: Opening