It was a set of protected mode device drivers that worked together to take advantage of advanced disk I/O features in the system BIOS.
It filtered interrupt 13h BIOS calls to the disk controller and directed them in the most efficient way for the system — either through the 32-bit interface with the hard disk controller or through the system BIOS.
Usually, 32-bit read could be safely enabled, but 32-bit write had issues with a number of applications.
Without it, if the real mode disk code (the Int 13h handler) was paged out, the virtual DOS machine would loop forever.
32-bit file access provided a 32-bit code path for Windows to directly access the disk bus by intercepting the MS-DOS Int 21H services while remaining in 386 protected mode and at CPU speeds, rather than handling the Int 21H services in real mode by MS-DOS.