One of the fundamental principles is maintaining a structurally sound stance. The Tai Chi Discourse states:
The root is in the feet;
Discharging is done by the legs,
The controlling power is in the waist,
And the appearance is in the hand and fingers. 
In health-based Tai Chi practice, correct stances are integral for ensuring joint health of the lower legs and for supporting the functioning of the leg muscles. In martial Tai Chi practice, stances are vital for ensuring one can effectively absorb and redirect the force of an opponent.
In the video below, Dave provides instruction in some of the basic principles of stance-work.
 Translation by Dan Docherty, http://www.taichichuan.co.uk/information/classics_lun_text.html