Generally it means that you are straight in the right places, and that you are making a nice shape with your body.
Look at the line that is formed along the bottom of her legs. If her back foot was flexed, it wouldnt be a nice smooth line and it would be ugly looking. That's what your coaches are talking about!
@Paint A Melody, that would be me as far as body description, in some skills my form gets my coach commenting that in others not so much.... I meant to quote you by the way, still trying to figure that out!