A travel dial was a wrist-worn teleportation device used for travel on the planet Marinus. Considering that the First Doctor was familiar with it but not with the planet, it must have been used on other planets as well. The Doctor referred to the technology as an acceptable method of travel.
The wearer merely had to twist the dial in order to effect instantaneous travel to the pre-programmed destination.
The Voord on Kandalinga had brought two travel dials with them. The Voord leader placed one on himself and the other on the Doctor so that he would take him to his spaceship. (PROSE: The Fishmen of Kandalinga)