An editor/librarian is a piece of software that helps you edit and store programs.
Digitally controlled synthesisers tend to have plenty of parameters you can tweak, but generally a rather inadequate interface with which to do so. Thankfully, as MIDI sysex messages let you send and receive programs, there's a way around this.
People wrote software for home computers that let you edit and store these programs outside of the synthesiser itself. This was pretty cost effective, as you'd likely have many synthesisers with cheap interfaces, and only one computer with a big screen, keyboard, mouse, and floppy disk drive. In effect, these programs worked much like the user interface of later software synthesiser plug-ins, only they were remotely controlling hardware synthesisers.
Editors/librarians: 4-Op Deluxe