What does sustained mean in dance?

Answer 1

If you're referring to the Ballet step 'soutenu', which means 'sustained' in French, then this step involved bending the knee of your supporting leg whilst extending your other leg along the ground in a tendu, in any direction, and sliding it along the floor to the next position (like a rond de jambe) before straightening your knee.

Being more general, this could mean lengthening each step or move so that they flow together more smoothly, or maintaining muscle tension in a step.