Basically it means casual but not too casual. You need a little spice in the sugar if you know what I mean. That's what dressy means. SO basically, jeans are ok, as long as they're good jeans. And, collared shirt, don't go overboard and wear like a suit. You'll look stupid.
Usher, Kanye West look at the stuff they wear when they're rapping.
They always dress nice, so does P. Diddy.
ex. Kanye West wears a polo and jeans, this is dressy casual.