Credence was a planet occupied by the Daleks. They wished to lure refugees from other worlds to Credence so they could be exterminated, so they set up a ruse. They maintained an appearance of working cities with people living in them, but it was a trap.
Captain Tillyard and the other captains of her planet made a deal with the Daleks to allow their refugees to live and work on Credence, completely unaware that it was a trap. The Third Doctor and Jo Grant tried to convince Tillyard that it was a trap, but she approached the Daleks on Credence, asking for asylum. They exterminated her, but Tillyard's ship was able to escape. (PROSE: Hide and Seek)