A utility once used for editing the contents of the ResourceFork of files (usually applications) on the Macintosh. Since the ResourceFork could contain many different data types - from text and settings, to icons and pictures, via sounds, colours, etc, ResEdit had to be a bit of a SwissArmyKnife. Indeed, it made a useful word processor - the 'styl' styled text resource gave 90% of what you need, and you could even have several in a file!

''"once used"? So what is used now? (I haven't touched a Mac for many years at this point, alas...)''

Not to be confused with !ResEdit which is still used for editing resource files on the Acorn line of machines, such as the RiscPC.