Temporal radiation is a type of radiation emitted by certain vessels equipped for time travel. Exposure to this radiation can cause individuals to repeat their last moment in time prior to being exposed.
The 31st century time travel pod discovered by the crew of the NX-01 Enterprise in 2152 emitted temporal radiation. Soon after the pod was brought aboard, the Tholians arrived to warn Enterprise of the radiation, hoping that Captain Archer to turn the time pod over to them. Archer refused, however, and had his crew continue to inspect the pod.
Tucker and Reed were the first to become exposed to the radiation after removing a panel from the time pod. As a result, they began reliving the same moment repeatedly. Although the particle density was too low to allow any lasting effects, Archer sealed off the launch bay where the pod was being held to prevent further exposure.
Later, Reed and Archer would go through the same experience of "deja vu" while attempting to load a warhead from a spatial torpedo onto the pod with the intention of destroying it before it could fall into Tholian hands. Although the Tholians were able to disarm the warhead, the pod itself would disappear after Tucker activated its temporal beacon. (ENT: "Future Tense")