Your character’s frame of reference, the sum of their backstory, talents, and the current fiction - everything on your sheet and what’s affecting you in the story. When a question arises about what you can do, have, or know, check it against your vantage: does it feel like a given, a stretch or a reach?.
Proper vantage opens up possibilities and gives the freedom to declare story details. Lacking it makes things more difficult, or even impossible.
You’re assumed to have the gear and skills that are a given for your vantage. This avoids detailed tracking and ensures PCs are competent where it makes sense.
Use the details on your sheet as creative inspiration - and limitations! - for roleplaying. They are player (not GM) tools. When unsure of your PC’s actions or thoughts, play towards or even against a relevant detail. Update details as the story unfolds, announcing the change and giving a vignette to give it weight and clearly convey the change to the other players.