Holocrons were ancient Jedi and Sith objects, that held both Jedi and Sith information, teachings, and holographic messages. During the time of the Republic, countless holocrons were stored on Coruscant.
Holocrons featured unlocked and locked modes. Some Jedi-related holocrons could only be opened through the Force as the Jedi employ it, while Sith-related Holocrons could only be opened by one who followed the teachings of the Sith.
One holocron was owned by a rebel named Kanan Jarrus, containing a message from Obi-Wan Kenobi warning any surviving Jedi about the rising of the Empire resulting the falling of the Jedi Order and the Republic that took place fifteen years ago. It also contained the locations of other Jedi Temples across the galaxy and lightsaber training practice techniques. It was by opening this holocron that Ezra Bridger proved Kanan's suspicions about Ezra's Force-sensitivity. Later, Kanan searched its information and learned that a Jedi Temple existed on Lothal.
Another holocron was hidden in the depths of the Malachor Sith Temple, accessible only if two Force-wielders employing anger worked in tandem to make their way inside and reach the holocron altar. Though not expressing the ability to open it, Maul revealed to Ezra that it served other purposes, such as opening the exit to the temple. It was eventually used to activate the temple's battle station mode, which Ezra refused to make use of, prompting him to retrieve the holocron with help from Kanan. After managing to keep Darth Vader from obtaining the holocron, the pair left Malachor with it, leaving Ahsoka Tano behind to face Vader in the process. This loss devastated Ezra and seems to have moved him towards the dark side, as he was later seen beginning to open the Sith holocron.
The holocron slowly corrupted Ezra by seducing him to the dark side and thus, making him even more powerful against the Empire. His now blinded master, Kanan, being concerned about the holocron's influence on Ezra, took it away from him. He later gave it to The Bendu as a "gift" after helping him see using the Force despite being blind.
After Kanan gave the holocron to the Bendu, Maul held hostage the Ghost Crew, except Ezra and himself. Maul demanded Ezra and Kanan to give him back the Sith holocron in order not to kill the rest of the crew. So, in order to save their friends, Ezra and Kanan, went back to Atollon in order to get back the holocron from Bendu. Having recovered it, they went to meet with Maul, who had managed to locate Kanan's Jedi Holocron aboard the Ghost, yet failed to open it. He sought to use the two holocrons in conjunction in order to unlock knowledge, which led to him and Ezra both seeing Force visions that were broken when Kanan intervened, resulting in the shattering of both holocrons.
The two destroyed holocrons were then placed in a room aboard the Ghost, where they remained inactive for some time. However, Maul-who had recovered a piece of one of them-was able to cause them to activate in order to lure Ezra to Tatooine. Ezra believed that he could use similar fragments to locate Obi-Wan Kenobi on the planet, but was tricked by Maul.