alt Hacker NewsApple's own documentation is a fine place to start - the first chapter of this book explains what resources were and how they were used:
https://developer.apple.com/library/archive/documentation/ma...
But the short version is that the Resource Manager provided a standardized way for applications to store a bunch of record-based data in a file - either as part of the application itself, or in files it created - and load those records on demand. The system used resources heavily to represent assets like code fragments, icons, dialog box layouts, or sounds, which could all be loaded on demand or automatically purged from memory when not in use.
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Palm OS borrowed the concept. This meant that you could edit apps on the device! https://blog.gingerbeardman.com/2022/10/07/fixing-bugs-using...
Emphasis on the memory management: because early Macs had only 128k/512k of RAM and a single floppy drive, dialogs and code fragments had to be constantly swapped in from application disks; and because early Macs had no paging, resources were accessed via handles.
You'd often have enough of a word processor resident in memory, to be able to work with a document disk in your Mac. But if you wanted to (say) print or run a spell-check, you simply didn't have enough memory to do so: so the System needed to purge resources, and load the requisite resources (code, dialogs) from the application disk. You'd be constantly swapping between user disks and application disks. Resources and handles were the way the System constantly shifted parts of an application in and out of the limited memory, tracking where they came from.