Used as a reply in certain contexts when someone has just said something quite verbose or confusing. "Bless you" can be used in the same way in this sense. Not all native speakers of English use it.
From the A Way With Words (waywordradio.org) radio show, Aug. 23, 2010:
Grant Barrett: "It's the second person singular indicative present of the verb 'to see', s-e-e."
(verb) to type XD
, which, if you tilt your head 90 degrees left, is the emoticon
of a face grinning so hard that its eyes are squinting.
me: You just ex-deed!
me: Stop ex-deeing.