(covers information from several alternate timelines)
A parametric frequency was a modulation to which a vessel's targeting scanners could be be set.
In an alternate timeline in 2374, the crew of the USS Voyager was able to remodulate their targeting scanners to a parametric frequency, which allowed them to destroy the torpedo launchers of the Krenim, an aggressive Delta Quadrant species which utilized chroniton torpedoes to negate the effectiveness of an enemy's shields, before the Krenim even fired their weapons. This timeline was negated in 2373 by Kes, who had been experiencing a series of backward time jumps as a result of the reactivation of dormant chronitons in her cells by exposure to a biotemporal field in 2379. (VOY: "Before and After")