The command reloads the image from
disk, so that it looks just like it did the last time it was saved
— unless, that is, you or some application other than
GIMP have modified the image file, in
which case, the new contents are loaded.
When GIMP reverts a
file, it actually closes the existing image and creates a new
image. Because of this, reverting an image is not undoable, and
causes the undo history of the image to be lost.
GIMP tries to protect you from losing your
work in this way by asking you to confirm that you really want to
revert the image.