It refers to the balance and the way you carry and represent yourself and this is one of the most important aspect of being a successful model. You do not need to have a posture like adriana Lima or Naomi Campbell, but it should be perfect enough to define you as a model or even an aspiring model. Your posture acts as a window that shows whether you have the “IT” factor or not and it is well judged by the agencies.
With your head held straight and high, you can give a look of sheer confidence and determination in comparison to a slouching posture that makes you a careless, casual and lacking attention. Of course there are always exceptions, but these are the most obvious signs.