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:
Depending on the type of portal, some are harder or easier to create. For instance, a portal that hangs in the middle of the air is far more difficult to create and takes more time, and thus only a very powerful mage can do so effectively. Other portals can be created easily enough to quickly whisk oneself away, like Yennefer did when she was trying to capture the djinn in Rinde.
However easy they are to use for mages, they do have their drawbacks: mainly if a portal is unstable or not set up correctly, it can quite literally tear one's body in half, killing them instantly (and the reason Geralt despises using them). It also appears that temporary portals can only help one travel so far: when some mages went to visit Vilgefortz, they commented on having to take 3 portals to get to him.
One can also set up a "blind" portal, but this is a gamble: if one doesn't know where they're teleporting to it could also very well lead to their death. However, under extreme circumstances, Yennefer managed to do this to get around what was perceived as an impenetrable barrier and ended up teleporting to Skellige.