A temporal tag was a 31st century technological device that allowed an object to be marked for retrieval from another time period.
In 2153, Daniels provided Captain Archer with temporal tags when he sent Archer and T'Pol back to Earth in the year 2004, to stop a temporal incursion by the Xindi. After defeating the Xindi, they were able to use the tags to mark everything that didn't belong in that time for transportation back to 2153. (ENT: "Carpenter Street")
In the final draft script of "Carpenter Street", the temporal tags were described as "tiny glowing points of light". The script detailed that "dozens" of them were to be inside "a small, futuristic device, [...] on the outside [of which] we see a series of switches".