A dimensional lesion was a hole in time and space which acted as a portal from one place in spacetime to another. Where/when lesions linked to was apparently at random. Beings, objects and radiation could pass through dimensional lesions from one side to another, but atmospheric gases apparently couldn't. Those who travelled through lesions suffered no ill effects, unlike users of vortex manipulators.
Lesions could be created when a time-travel-capable vehicle like a TARDIS collided with another craft. If the time-travel-capable transport arrived at the location of the craft which it had "bumped" into some time later, more lesions could open there. They could, however, be closed with an improvised device and detailed knowledge of the event, as the Eleventh Doctor did. (GAME: The Gunpowder Plot)