The klaxon was an audio signal designed to notify personnel located on a planetary body, or aboard a starship or starbase, that they were under red alert.
In 2152, Malcolm Reed began to develop a new security protocol, which he dubbed "tactical alert." In conjunction with the protocol, he came up with several possible alert signals, so designed to get the crew's attention. When he proposed each of two alert signals, a klaxon and a buzzer, with Charles Tucker, Tucker described them both as sounding "like a bag full of cats." The klaxon Reed ultimately ended up using was not retained after Enterprise escaped the mind-affecting radiation influencing every non-Vulcan member of the crew including Reed; though Captain Jonathan Archer approved the new protocol, he mandated a less annoying alert signal. (ENT: "Singularity")
While under the influence of DaiMon Bok's thought maker in 2364, Captain Jean-Luc Picard began experiencing flashbacks of the time during his involvement in the Battle of Maxia. He explained to Doctor Beverly Crusher that in these intense experiences, he found himself on the bridge of his old ship, the USS Stargazer, where he was surrounded by fire, thick smoke, and the sound of the klaxons, moments before giving the order to abandon ship. (TNG: "The Battle")
Different ships had different klaxons that sounded during alerts. (Star Trek: The Original Series; Star Trek: The Animated Series; Star Trek: The Next Generation; Star Trek: Deep Space Nine; Star Trek: Voyager) Although these klaxons typically sounded during such alerts, they could be silenced at the discretion of the vessel's commanding officer. (TNG: "Encounter at Farpoint")
The klaxon was heard several times in many Star Trek: The Original Series, Star Trek: The Animated Series, and even in many Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager, and Star Trek: Enterprise episodes. This sounds off as a red light begins blinking throughout the ship or station. Examples include "The Doomsday Machine" and "The Best of Both Worlds", as the captain orders a "red alert" in reaction to a hostile event, such as the ship or station being fired upon by a hostile ship.