Instead of a regular bass string, where you have a round wire wrapped around a thinner inner core string ( or flat metal or nylon tape around the same for flats and tapes respectively) you have an inner core of several very light strings ( like a bunch of guitar .008 or .009 strings). They have a low tension rubbery vibe and they sound a bit different but not in any bad way. They were expensive and hard to find, back when Rotos were kind of unusual vs Fender, LaBella or GHS.