The Hada-Juban is an the first article clothing when wearing a formal kimono.
Hada means skin. The Hada-Juban is made of 100% cotton. It helps to keep the kimono from getting soiled. Augberginefluer provides the following advice for wearing Hada-Juban under Yakata in her blog "Japan Now & Then":
The photos below are of the front and back of the Hada-Juban taken retrieved from Japanese Lifestyle).
1) One-Piece Hada-Juban for Yukata
This is worn over your underwear to prevent the yukata from sticking to your skin, and though I went years without bothering to wear one, it is much cooler if you do. The best is a 100% percent cotton or linen full body shift. Avoid ones with a polyester skirt. Sales assistants will try to talk you into a large size if you are a gaijin to move the merchandise, but unless you are large, get the medium because the extra material of the skirt will make it hard to walk because it tangles around your legs. Appropriate underwear is a sports bra and panties for a woman and V-neck T-shirt and boxers for a man; men do not need a hada-juban.
Sites of interest: