A system enabler contains resources for a specific system to run a particular version of MacOS that was designed prior to the hardware being released. Since it isn't documented anywhere(*), it likely contains the following:

-A memory map of the system being booted

-Drivers for any special hardware like audio, video, system chipset, etc.

*It surprises me that after all these years, nobody has disasembled and done a low-level analysis of the Classic MacOS system "kernel", ROM, and system enablers. To this day, articles about the OS say the boot process is shroulded in mystery and that pre-Open Firmware Macs can only boot MacOS before kick starting another OS.