A powerful and rare magic, rifts are the openings – seemingly torn right into the air – that allow the magic user to step from one location to another at some distance. Rift-users must be powerful in order to hold that gap open; it’s been likened to lifting a huge, heavy stage curtain and holding it up as one walks beneath.
Some magic users, like Bran, are able to jump between worlds with rifts, but it’s such a rare skill that some magical philosophers believe it to be a different skill entirely.
There are some who believe creating rifts destabilizes the world in which they are made, though at such a slow rate that this is mostly a theoretical concern. Mostly.
Note: rift-makers are distinct from world-walkers, who are able to travel from world to world without tearing holes.