@stevenyc Hi. Imagine a curtain hung vertically from the ceiling of your listening room down to the floor (like theatre curtains on a stage), at a certain distance from your listening chair. If when it hangs down it just touches the front of your speakers, that would be on the plane of the drivers.
Soundstage. Some speakers project more forward soundstages, and some more laidback. Some soundstages are deep, some quite shallow. Other acoustic factors (beyond the speakers) also enter into this.
Forward: the curtain now marks the front of the soundstage, and it appears to be quite close to you, closer than the front of the speakers.
Laid-back: the curtain appears to be hanging somewhere behind the speaker cabinets.
Deep: soundstage expands all the way to the front wall (front wall=the wall behind the speakers), or even appears to be outside the room, beyond the front wall.
Shallow: etc. etc.
Curtain is not the best analogy because the plane is visually and acoustically transparent, but perhaps that gives you an idea.