Shield modulators were technological components of a starship's defensive systems, responsible for maintaining and adjusting shield harmonics.
In 2368, Lieutenant Commanders Data and La Forge adjusted the USS Enterprise-D's shield modulators in order to allow the ship to push through the gravitational wavefronts of a black cluster, as part of their investigation into the destruction of the SS Vico. (TNG: "Hero Worship")
In 2371, Chief Miles O'Brien utilized the shield modulator of the USS Defiant to adjust the ship's shield harmonics to make it appear as a Kobheerian freighter to long-range sensors during a covert mission to Cardassia to rescue Major Kira. (DS9: "Second Skin")
In 2375, the Borg locked onto the shield modulators of the Delta Flyer, which was being used by the crew of the USS Voyager in an attempt to rescue Seven of Nine from the Borg Unicomplex. This allowed the Borg to adapt mere moments after Lieutenant Paris, piloting the Flyer, remodulated the frequencies of the multi-adaptive shielding he was using in an effort to stay invisible to the Borg sensors. (VOY: "Dark Frontier")