The verteron array was a powerful directed-energy device based on Mars as of the mid-2150s, used to redirect comets toward the ice caps for the purpose of terraforming.
In 2155, John Frederick Paxton, leader of the xenophobic group Terra Prime, hijacked the array to further his cause of Human superiority. Using the array to fire on Earth's moon as a demonstration of its power, Paxton threatened to destroy Starfleet Command unless all aliens evacuated the solar system.
When Enterprise NX-01 attempted to stop him, Paxton used the array to fire on Enterprise, critically damaging it. Malcolm Reed noted that the array was only firing at two percent power, and that Paxton could have used the array to vaporize them. Eventually, Captain Archer was successful in leading a team to infiltrate Paxton's base and stop him, but not before Paxton locked in the firing sequence for the array. Fortunately, although they were unable to stop the array from firing, Commander Charles Tucker was able to change the target of the verteron array, which fired harmlessly into the Pacific Ocean. (ENT: "Demons", "Terra Prime")
According the the DVD audio commentary for "Terra Prime", the writers originally envisioned the verteron array to simply be a very powerful laser beam. They soon realized, however, that a laser beam would not be able to reach the targets depicted in the episode in such a short amount of time, since a laser beam would be limited by the speed of light. For this reason, they decided to use verterons, which are fictional particles.
In the final draft script of "Demons", the verteron array was described as "a sprawling complex embedded in the Martian terrain" and "a towering row of automated high-tech particle cannons, all aimed at various points of the sky."