In the fictional universe of the first Transformers animated series, Vector Sigma is the "mega-computer" that gives sentience ("Sparks" in later parlance) to non-sentient robots, and is used by the Quintessons to endow their robotic creations with the true life that eventually leads to their rebellion. Vector Sigma attests that "before Cybertron was, I was." Subsequent fictions, such as Dreamwave's Transformers, would establish that Vector Sigma is a mainframe used by Primus, the creator god of the Transformers.
Vector Sigma requires a special circuit key to activate itself, which completes its power circuits. Additionally, should the key be lost, Vector Sigma's first-generation creations share his computation matrix, and are hence able to interface their power supply with it, reactivating it at the cost of their own life. The key itself possessed the ability to "de-energise matter," converting organic substances into "technomatter" (a mechanical and metal facsimile of itself).
Read more about Vector Sigma: Generation 1, Beast Machines, Beast Wars Neo, Robots in Disguise, Fun Publications, The Omega Terminus, The Terminus Blade, Cyber Missions, Transformers Animated, Transformers: Prime