They are a kind of necklace, commonly made of knit fiber (nylon or polyester), from which you can hang a key or a card, commonly used to hang identification/access cards at some places of employment. Aboard a ship, it allows you to tote your sail-and-sign/cabin passkey with you without any need for wallet, purse, or other means to keep track. Especially great for kids, who find it so easy to misplace same.