When something's sanitary, it's extremely clean. It's important for hospitals to be sanitary.
The adjective sanitary is useful when you need something stronger than just "clean." Sanitary implies the absence of germs as well as a clean appearance — you probably hope that your favorite restaurant's kitchen is sanitary, and your visiting friends will appreciate your bathroom being sanitary as well. The origin of sanitary is the Latin word sanitas, or "health," from the root sanus, which means both "healthy" and "sane."