The Macintosh was intended to be different in many ways. One of them was its file system, which was designed for each file to consist of two forks, one a regular data fork as in normal file systems, the other a structured database of resources, the resource fork.
Resources came to be used to store a lot of standard structured data, such as the specifications for and contents of alerts and dialogs, menus, collections of text strings, keyboard definitions and layouts, icons, windows, fonts, and chunks of code to be used by apps. You could extend the types of resource supported by means of a template, itself stored as a resource, so developers could define new resource types appropriate to their own apps.
Apple’s engineers developed a resource editor that quickly became one of the best-known apps on the Mac: ResEdit, last seen in version 2.1.3 way back in 1994.
I missed the ResEdit era by a few years, but for many people, it was one of the things that made Mac OS so special.