The Focus was a component of the Imagineum which was necessary for it to function. It resembled a pendant with an image of an alien face and a sunburst on it.
After Marwood lost the Imagineum to the Celestial Toymaker in a game of canasta, his adjutant Felix stole the Focus, intending to return it to the Knights Templar. He gave it to Izzy Sinclair and Maxwell Edison to get it out of the village. After they met the Eighth Doctor and told him what happened, the Doctor instructed Izzy to find an entry for the Focus in his copy of Jane's Alien Artefacts in the hopes of identifying it. Later, the Doctor was forced to return the Focus to the Toymaker to save Izzy's life. (COMIC: Endgame)