Spacefolder ships use the Holtzman effect to bend spacetime. Computers were used to find safe routes before the Butlerian Jihad. Without computers, guild navigators use their prescience to find safe routes. Spice doesn’t allow spacetime bending, just navigating through bent spacetime without a high fatality rate from flying into stars.
If you watch really closely in Dune P1, you can see another planet (presumably Selusa Secundus) through the center of the navigator ship. I don't know how it functions mechanically in his version, if they're navigating wormholes somehow or controlling them, but they have some form of wormhole mechanism shown in the series.
Literally navigating. It’s apparently hard to travel long distances that way without running into stars and whatnot. They used to use computers to chart safe courses. Then after the butlerian jihad, most long distance travel stopped due to the high rate of “oops”. It only resumed when the guild was able to use prescience to know which routes wouldn’t result in everyone dying.