Kirk points out the blurry distortion of a cloaked Bird-of-Prey
A cloaking device was a form of stealth technology that uses selective bending of light to render a starship or other object completely invisible to the electromagnetic spectrum and most sensors. Cloaking technology was encountered in varying forms over the centuries.
One of Starfleet's earliest encounters with cloaking technology was in 2151, when the Suliban Cabal used a cloak to covertly approach Enterprise NX-01 and kidnap the Klingon Klaang from the ship's sickbay.
By the 23rd century, practical invisibility was considered only theoretically possible as the amount of power required would be enormous. In 2266, the USS Enterprise encountered a Romulan Bird-of-Prey that used a cloaking device to cross the Romulan Neutral Zone and wipe out several Earth Outpost Stations.