Stance refers to the way we stand on the skis and a good stance on skis is critical for balance and performance. Stance is not a static notion, because our relationship to the skis and the slope changes throughout the turn, but rather stance describes an average, a target, a "home base" we tend to find ourselves in more often than not, especially between turns.
While the stance should be overall athletic, a narrow stance promotes mobility while a wide stance promotes stability, too many error on the side of stability, forgetting that skiing is more like dancing than sumo wrestling, see Stability with mobility and Skiing - the wide stance debugged.
Some aspects of stance, to consider:
Here's a tall stance, typical for skiing green runs - contrast the apparent immobility with the more dynamic blue stance above:
And here's a typical expert stance, quite low, while carving black runs:
While you may think this kind of stance puts a big strain on the legs, in reality there is no weight on the legs right now, as the skier got a lot of performance from the previous turn and is Floating between turns.
Talks and sessions to work on stance: