The Progenitor device was a piece of Dalek technology designed to create new Daleks in the event of the Daleks about to go extinct. It stored pure Dalek DNA, and when properly utilised and fueled, could both reproduce fully-grown Dalek mutants and functional battle armour.
Thousands of such machines were created by the Dalek Empire. Eventually, all but one were lost. The last of these devices was used to restore the Dalek race by establishing the New Dalek Paradigm. (TV: Victory of the Daleks)
During the Last Great Time War, the Daleks used them to quickly populate the areas of the universe they conquered to make it impossible for the Time Lords to properly cope with their numbers and giving them new fronts to fight. They planned to create a new paradigm of Temporal Weapon Daleks armed with Temporal Cannons that could erase their victims from history. However, the new planned paradigm was never deployed because the Eternity Circle was stopped by the War Doctor and Borusa. (PROSE: Engines of War)
However, a single Dalek saucer, badly damaged and severely depleted, escaped destruction after an encounter with the Doctor, falling through time to 1941. The three Dalek drones on board were the last three Daleks in existence. They detected the signal of the last Progenitor device, acquired it, and installed it in their saucer. They were unable to use it to create new Daleks, however, as the Progenitor did not recognise them as Dalek. However, it had been programmed to recognise the Time Lord known as "the Doctor" as the enemy of all Daleks. If they could somehow acquire the Doctor's testimony that they were indeed Daleks, the progenitor would accept this as proof and would begin functioning.
To accomplish this, the three Daleks created an android called Professor Edwin Bracewell in London. While one Dalek remained in Earth's orbit with the Progenitor, the other two feigned to be Bracewell's simple robots, called "Ironsides". They eventually tricked Winston Churchill into calling the Eleventh Doctor. After arriving, the Doctor became so confused and frustrated that, in a fit of anger, he struck an "Ironside" repeatedly, addressed them as "Daleks", and briefly recounted his violent history with them. The Daleks transmitted this testimony to the Progenitor, which recognised the Doctor, and duly accepted the command to activate. Shortly afterward, the first five individuals of the New Dalek Paradigm were created. The first action of the New Paradigm Daleks was to disintegrate their three "unclean" forebears, who willingly offered themselves for extermination. The New Paradigm then escaped with the Progenitor through a time corridor to the future and successfully created a new empire, while the Doctor and Amy Pond remained on Earth to defuse the Oblivion Continuum bomb inside the android Bracewell. (TV: Victory of the Daleks)