Complex sentence: The "stage 1.5" image contains file system drivers, enable it to directly load "stage 2" from any known location in the filesystem, for example from/boot/grub.
Split into simpler sentences: The "stage 1.5" image contains file system drivers. This enables the "stage 1.5" to directly load "stage 2" from any known location in the filesystem, for example from/boot/grub.