"Dress to impress," typically, means no denim (sorry, Dean), sweatshirts, t-shirts, hats, or sneakers. What it means for girls is harder to define since it's used most often when describing the dress code at clubs and what most girls wear to clubs probably isn't appropriate.
Use common sense -- don't look like a slob, but it's not a white tie affair, either, and it's not even as formal as semi-formal.