This is useful for when culling photos from a large shoot.
Flags can be in three states- Flagged (P), Un-flagged (U), Rejected (X).
So you might- go through the photos and mark all the wanted photos as [P] Picked/Flagged.
(Note: the 'tilde' key [`] beside numberic  will flag Pick as well as the [P]. So with three fingers on [`] you can pick and star 1 or 2 without moving the left hand. And with Auto-advance on, move through photos faster. )
Now the "Refine" command downgrades the flagged status for all the photos in the Grid-
Flagged photos become Un-flagged,
Un-flagged photos become Rejected.
Now you can select the 'Rejected' images and cull (Delete?).
You can do this process again to really 'narrow down' the photos you really want to keep.
When DNG files are created (if you use them!) there is a hash number stored in the DNG file.
When you develop a DNG in Lightroom, Lr 'looks at' the DNG files and checks the hash. If even a single bit of the image data has changed (corrupted) it would no longer match the hash and Lightroom can alert you. This Menu command will also check the DNG hash whenever you want to check your DNGs.
So you "Validate" the source image data.
Other file formats cannot be validated in Lr in this manner.