A jamming signal or jamming frequency was a transmission capable of disrupting various ship's systems, including communications, sensors, and transporters.
In 2152, the crew of a Mazarite ship that was pursuing Enterprise NX-01 utilized a jamming signal to disrupt the Starfleet vessel's communications and sensors, thereby preventing the Enterprise crew from requesting assistance from a Vulcan ship. Ambassador V'Las, whom the Enterprise crew was attempting to safely transport back to the Vulcans, suggested transmitting on a little-used diplomatic frequency, which she suspected the Mazarites were unaware of. In order to conceal the approach of the Vulcan ship, Lieutenant Reed, Enterprise's tactical officer, was able to deflect the Mazarites' jamming signal, disrupting their sensors as well. The plan was successful, and the Vulcan combat cruiser Sh'Raan was able to come to the Enterprise's assistance, driving the Mazarites off. (ENT: "Fallen Hero")
In 2374, The Doctor, EMH of the USS Voyager, managed to remodulate one of Seven of Nine's Borg implants to emit a jamming signal, successfully disabling a neural interface with which she and the rest of the Voyager crew had been outfitted by a Hirogen boarding party. This allowed Seven to subsequently disable Captain Janeway's interface, who then deactivated the interfaces of the entire crew, allowing them to retake Voyager from the Hirogen. (VOY: "The Killing Game", "The Killing Game, Part II")
The high-energy drill utilized by the Romulan mining vessel Narada generated a jamming signal as a side-effect of its operation. In the alternate reality created by Nero's temporal incursion, this required a team from the USS Enterprise to space jump from a shuttlecraft to the drilling platform in 2258 in order to manually disable the drill, allowing the Enterprise to re-establish transporter control and mount a rescue mission to Vulcan. (Star Trek)
In 2263 of the same alternate reality, just before the Battle of Altamid, Krall's swarm drones seemed to jam Enterprise's communications, which Nyota Uhura alerted the rest of the command crew to. However, this was not intentional; these were VHF radio signals which the drones were using to communicate, which Enterprise's former crew exploited during Krall's final assault on the Federation. (Star Trek Beyond)