The dress is a two-piece set designed for children, likely girls, and is displayed on a mannequin. It includes:
Top:
A long-sleeved black shirt with a crew neck. It appears to have some graphic design or pattern on the front, though the details are not very clear.
Bottom:
White and light grey striped pants with suspenders. The suspenders are attached to the front of the pants with buttons and are adjustable. The pants have a wide-leg style and feature decorative buttons near the waistline.