"Upright torso" refers to a straight and vertical position of the upper body, where the head, neck, and spine are aligned without leaning or slouching. Full definition
The wider your stance, the more upright your torso can to be. (getthisstrength.com)
This enables proper hip alignment, and therefore a more upright torso. (kingofthegym.com)
Also, the front squat encourages or, rather, insists on a much more upright torso position which means that your lower back is under much less stress compared to back squats. (fitnessandpower.com)