Scrip in FO76 is a special currency that you can obtain by trading in legendary weapons & armor. To do this, you need to deposit them into special containers found at train stations, or at the purveyor’s location. To see the location of the purveyor, watch the video I’ve embedded. A lot of other videos and images are outdated so that’s why I made a new one.
Purveyor sells different levels of legendary items for increasing scrip costs. It’s simply a system designed to let you shuffle through legendaries a bit faster than you normally would.