Portals are essentially teleporters. Some appear to be more or less permanent physical structures, while others are conjured at will by mages. They allow individuals to travel from one place to another and in some cases, to a different place in time. There are several physical portals in the Witcher universe:
In the short story "The Last Wish", Yennefer makes ample use of spontaneously conjured magical portals to transport herself, Geralt and Dandelion about the town of Rinde. Geralt himself is quite suspicious of these portals and uses them only grudgingly.
In the video games, Geralt avoids travelling via portals if he can help it, if not, he almost always openly displays his displeasure in using them.