The machine brain was a powerful device created by the Daleks in the desert of Skaro. It was made of a new experimental metal which was invulnerable to all but nuclear weapons, and armed with death rays which were lethal to Daleks but not to other life forms.
The scientists who constructed the machine brain ordered it to produce great quantities of Neutronium. This was their last order as the machine destroyed the scientists. Knowing that if enough Neutronium were produced it would explode and destroy Skaro, and with no way to stop the machine brain, the desperate Dalek Emperor sent a Dalek to Earth to contact the First Doctor for help. The Doctor ordered a Dalek suicide squadron to attack the machine with explosives that would not set off the Neutronium, but this had no effect. He then approached the machine himself, and was unaffected by the death rays, since they were intended only to harm Daleks. He switched off the power, leaving the machine silent. (PROSE: Doctor Who and the Daleks)