This article is from the Ballet and Modern Dance FAQ, by Tom Parsons email@example.com with numerous contributions by others.
Placement is, roughly, alignment of the body. Becoming properly
placed means learning to stand up straight, with hips level and even,
shoulders open but relaxed and centered over the hips, pelvis straight
(neither protruding nor tucked under), back straight, head up, weight
centered evenly between the feet. This posture is frequently described as
"pulled up," but it is also a relaxed posture; you aren't tensed up like
a soldier standing at attention. (A teacher once said you should imagine
that you are suspended by a thread attached to the top of your head. This
suggests both the "pulled-up" and relaxed aspects of good ballet posture.)
And as you dance, you seek to maintain this posture except when the step
requires something different, like the slight forward arch of the spine
that accompanies an arabesque.