The directional unit was a TARDIS component. It helped in navigation to specific places. The First Doctor stole the directional unit from the Monk's TARDIS so he could steer his own TARDIS to Kembel. However, the unit exploded when the Doctor tried to take off, and was irreparably damaged. (TV: The Dalek's Master Plan)
Without his directional unit, the Monk was only able to go to random locations. He eventually arrived in 11th century Ireland, becoming the Abbot of the Abbey of Kells. He used the monks to help him draw a new directional unit for his TARDIS. (AUDIO: The Book of Kells)