With the nametag you can see how many locks you have on your chest. But this only works if you are the owner of the chest. You can see how many crowbars a robber needs to open your chest. With the Crime-Texturepack the nametag has another texture.
The usage is pretty simple: Rightclick on a chest and you see the number of locks on the chest in the chat as well as the remaining uses. You can use the nametag 8 times.
You can buy this useful item at the technician in the tv-tower, who sells nametags for 80 dollars apiece. To buy you need to use the ID 421. It isn't possible to sell this item at shops.
Price: 80$ / ID: 421