Cerebro is an advanced piece of technology Professor X uses to find other mutants. It uses the Mutant Power Circuit to amplify Xavier's powers. Cerebro is also connected to the Xavier Mansion's defense systems.
When Kitty met the X-Men the mansion got a red alert, meaning a mutant threat. The X-Men got into two jets and left.
Meanwhile, they were being watched by Magneto and Juggernaut. Together the two destroyed the Mansion's defenses. Kitty and Xavier watched on monitors while he explained that Magneto is the master of magnetism while Juggernaut Xavier's step-brother. Too worried to notice, Kitty phased through Cerebro's defensive circuits allowing the two evil mutants to enter.
Xavier scanned Magneto's mind and learns that he came for Cerebro's Mutant Power Circuit. Xavier handed the circuit to Kitty just as the two attackers entered. Using her powers she ran off with Magneto in hot pursuit while Juggernaut confronts his step-brother. Magneto used electrical cables to shock Kitty into dropping the circuit and phasing through the floor. He picked up the circuit and returned to Juggernaut.
First version of the device outside the comics.