When working with a script, an actor knows a lot about the scene they are about to perform. They know where the scene takes place and who their character is. They know the relationship between themselves and the other characters in the scene. They know what they are going to say. And if they are properly prepared, they know what their character wants and what actions they will do to try to get it.
When an improviser begins a scene, they know none of these things. They face a stage that could become any setting they can imagine. They can play any character they choose and so can their scene partner. Their choices are infinite. So at the beginning of an improvised scene, the most important thing they must accomplish is to decide on the circumstances of the scene. And the most important tool for deciding those circumstances is agreement.
Simply put, an improviser must agree to all facts and circumstances that their scene partner establishes via dialogue, behavior or action. If I say that I’m a plumber, you must agree that I’m a plumber. If you act like you are in car, I must accept that. If I say that we are in an airport bar, set down your luggage and grab a drink.