Acceptable:
The phone has multiple marks, dents and scratches on the frame. The screen has visible scratches as well, but it is uncracked and intact. Put a case on this phone, and it will serve your daily needs just as well as any phone would.
 
Good:
The phone will have some visible signs of wear, such as scratches and marks on the frame and the screen. The scratches on the screen are hard to notice when the phone is turned on.
 
Excellent:
The phone you'll get is in excellent condition. It may have a couple of very light scratches on the frame and the screen, but no notable signs of wear.