A helmet that fails to protect the head, face and neck in the event of a crash or during regular conditions is of no use to a rider. A DOT sticker (US Department of Transportation certification) found on the back of a helmet indicates that the helmet meets motorcycle helmet safety standards.
The DOT standard requires that a helmet soak up a large amount of impact energy, prevent most types of penetration and have a fastening system that holds up to significant force.
When a motorcycle helmet manufacturer passes the DOT standard then the helmet becomes a DOT approved motorcycle helmet and is allowed to present a DOT sticker.
Some riders maintain that there are safe helmets that have passed another type of motorcycle helmet test and meet standards such as Snell Foundation certification. However, most experts agree that a DOT approved motorcycle helmet is a highly recommended means of ensuring motorcycle safety.