What is the Preboot Folder?
The Preboot volume is a new system partition that manages boot, the partition is created during the update to macOS High Sierra to support booting to an APFS volume(The one named Macintosh HD).
APFS is a new filesystem created by Apple, and it makes a lot of changes(link to Wikipedia article about APFS), and one of them is the addition of the Preboot volume, which is required to boot APFS, so if you delete it, you can't boot!