Multitronics were an advanced computer system, an evolution of the older duotronic system. A multitronic computer could theoretically be advanced enough to run an entire starship by itself.
Multitronics, though very advanced, were a dead-end for most civilizations in the Milky Way, because it was nearly impossible to create a stable operating system for a multitronic system. In the United Federation of Planets, Doctor Richard Daystrom attempted to solve this problem by using his own memory engrams as a basis for the M-5 computer, but the resultant system was revealed to be insane after it was tested on the USS Enterprise (NCC-1701) in 2267. The Federation ultimately abandoned multitronics in favor of isolinear technology. (TOS episode: "The Ultimate Computer", SCE eBook: The Future Begins)
The Kropaslin were the only civilization known to have created a stable multitronic computer core. They used it to store their most secure data, as the rarity of multitronics meant that virtually no one could tap into it remotely, as very few people knew how to interface a multitronic system with a more conventional isolinear/bioneural one. Only those engineers that had created it knew how, and they were kept with the computer, well away from outside contact-- the Kropaslin did not want to lose any of their technological secrets, and thus economical advantages.
When Fleet Admiral Alynna Nechayev needed someone to tap into the Kropaslin computer and steal the location of a Breen frigate in 2375, she called upon Captain Montgomery Scott. As chief engineer of the original Enterprise, he had supervised the installation of the M-5 unit, and he knew how to interface multitronics with more conventional circuitry. Thanks to his time-displaced status, he was the only Federation citizen with the needed knowledge still alive. (SCE eBook: The Future Begins)