Inhibitor chips were biochips that were engineered into the clone troopers of the Grand Army of the Republic.
The chips were originally intended by Jedi Master Sifo-Dyas as a safeguard against rogue Jedi. However, after his death at the hands of the Sith, they retooled the chip to force the Clones to slaughter the Jedi upon command. When Chancellor Palpatine, secretly the Sith Lord Darth Sidious, issued Order 66 at the end of the Clone Wars, this caused the inhibitor chips to forcibly brainwash the Clones into turning against the Jedi, leading to the defeat and near destruction of the Jedi Order and rise to Palpatine's Galactic Empire.
Tup under the influence of his chip
The chips were surgically implanted into the brains of every single clone embryo past stage three of development. Allegedly to prevent clone troopers from being overly independent and aggressive, their true function was intended by Jedi Sifo-Dyas to be a safeguard against control by rogue Jedi or Sith. However, the covert involvement in the cloning program of the Sith Lord Tyranus—known publicly as Count Dooku to most in the galaxy—ensured that from the very beginning of the Clone Wars, the chips would play an integral part of the Sith conspiracy to control the clones and guarantee them capable of killing their Jedi Generals in accordance with Order 66.
Sifo-Dyas was the only Jedi to know of the chips' existence. Upon his death, only those who were involved from the beginning of the cloning program, such as the Tyranus, his Sith Master Sidious and such key Kaminoans as Lama Su and Nala Se knew of the chip's true purpose, and they sought to suppress any knowledge of it. The chips were subsequently supplied by Tyranus, whose involvement remained a closely guarded secret by the Kaminoan leadership. The Kaminoans themselves apparently believed Tyranus to be a secretive member of the Jedi Order, even though through his directives they actively kept the nature of the chips from being known by anyone else.
Around 20 BBY, the chip of the trooper "Tup" malfunctioned, causing his Jedi-killing protocol to activate prematurely. Another clone nicknamed "Fives" ended up discovering the truth about the inhibitor chip, but he was framed for attempting to assassinate Supreme Chancellor Palpatine and was shot by one of his fellow clones before he could reveal it.
It was only towards the end of the Clone Wars that the Jedi learned the elusive figure they had identified as Tyranus was in fact Count Dooku. This led to the realization that the Sith had been involved in the cloning program from its inception, although they knew not to what purpose. They chose to keep this knowledge a secret from the Senate, as they thought it would destabilize the Republic and undermine the war effort. Instead, they resolved to take the only path they could see available, which was to play the game they now knew to have been orchestrated by the Sith, and bring an end to the war as quickly as possible.
Not too long after betraying and orchestrating the death of Dooku, Sidious turned the Chosen One Anakin Skywalker to the dark side and made him his third apprentice. With Anakin, now named Darth Vader on his side, Sidious instructed the Clones to execute Order 66. The issuance of Sidious's command activated the inhibitor chips, which then brainwashed the overwhelming majority of Clones into turning on the Jedi. The Jedi found themselves being betrayed and killed by their Clone Troopers who had served under them for the past three years. The Clones and Vader proceeded to kill and nearly destroy the Jedi, with only a handful of Jedi surviving the onslaught, and even a few clones managing to resist the order.
Concept art of a chip
Inhibitor chips first appeared in The Lost Missions season of Star Wars: The Clone Wars in the episode "Conspiracy," the beginning of a four episode arc written by Katie Lucas.