Dog tags are tags worn by army personal used to identify a persons body if killed in action or for medical details like blood type if in need of treatment. They are usually made from metal with names, date of birth, blood type and regiment in the army etched onto it. Two are used so they can be broken easily.
They were constanly seen and mentioned in the film like when Captain Miller removed Private Adrian Caparzo's tags and when Lieutenant Dewindt handed Miller a bag of dog tags. They are not bullet proof, however, proven by how Private Richard Reiben commented on a soldiers dog tag saying "look at this poor bastard" referring to a bullet hole with blood around it.