An exploding shield generator of Deep Space 9
A shield generator was the physical component of starships, space stations, and other installations, utilized for the generation of protective energy screens with which to defend against outside attacks. In combat situations, it was common for a vessel to target weapons fire at an adversary's shield generators, in an attempt to disable or weaken their shield strength.
Overview
In 2154, Degra was able to knock out the shield generators of a Xindi-Reptilian warship, allowing the crew of the Enterprise NX-01 to target the warship's weapons. Degra subsequently destroyed the warship, to prevent its crew from reporting the fact that Degra had entered into an alliance with the Enterprise crew. (ENT: "The Forgotten")
That same year, Commander Shran supplied the Enterprise crew with the coordinates of the shield generator of what appeared to be an Andorian battle cruiser which was attacking Enterprise. However, upon firing weapons at the coordinates, it was discovered that the attacking vessel's shields were still at maximum power. It was later determined that the vessel was not, in fact, Andorian, but was rather a Romulan vessel disguised as an Andorian ship. (ENT: "Babel One")
Later that year, in an attempt to retrieve Ambassador Soval, who had been abducted by Shran, Lieutenant Malcolm Reed, Enterprise's tactical officer, fired upon the shield generator of Shran's vessel, in an attempt to use the transporter to retrieve Soval. (ENT: "Kir'Shara")
In 2367, after an encounter with a Borg cube, the crew of the USS EnterpriseNCC-1701-D experienced difficulties in keeping the auxiliary aft shield generators online, although Commander Shelby estimated that they could be back to full operating capacity within three hours. (TNG: "The Best of Both Worlds, Part II")
Later that year, while engaged in combat with a Cardassian vessel, Captain Picard ordered Lieutenant Worf, the Enterprise's tactical officer, to concentrate weapons fire on the Cardassians' weapons and shield generators. The tactic was successful, and the Cardassian ship was forced to stand down. (TNG: "The Wounded")
In 2368, Enterprise engineering teams installed five new shield generators at the Human colony on Moab IV, in order to fortify the structure's structural integrity so that it could withstand the gravitational forces of a stellar core fragment passing near the planet. (TNG: "The Masterpiece Society")
In 2369 Major Kira Nerys suggested a Bajoran tactic to take over the shielded Federation freighter Norkova and overwhelm the mercenaries and Rao Vantika. During the Occupation of Bajor the Bajorans terrorists use to disable the shields of orbiting Cardassian freighters by taking their vessel close to the freighter and overloading their shield generators with directed phaser fire. (DS9: "The Passenger")
The same year Miles O'Brien ordered the computer to restart all shield generator subsystems and received a 98.3% efficiency. With this task, O'Brien tried to show Commander Sisko the "strange behavior" of the computer voice. (DS9: "The Forsaken")
In 2372 during the Klingon attack on Deep Space 9, Gowron ordered Martok to concentrate all fire on DS9's shield generators. They destroyed at least two of them, which was reported by Dax later. (DS9: "The Way of the Warrior")
During the Second Battle of Deep Space 9, the Dominion destroyed Deep Space 9's shield generator. (DS9: "Call to Arms")
The shield generators on the USS Defiant were configured differently from those on its predecessor, in order to improve protection against the Breen energy dissipator. (DS9: "The Dogs of War")
One, an advanced Borg drone with 29th-century technology, modified the shield generators of the USS Voyager in 2375 so that they could break free of a Borg sphere's tractor beam. (VOY: "Drone")
Background
The phrase "shields up," which saw widespread use in Star Trek, probably had its origin in classical warfare, when infantry would raise their shields to defend against incoming arrows or other projectiles. The order would be given when it was time for the soldier to raise his shield from the carry position.