Verbalization of '...' which is the written form of a pause in speech.
Verbalizing the written substitue for a short silence adds a level of sarcasm to the comment (or absence there of.) When used in place of a glare to indicate ignorance, it has the effect of softening the regection.
Can also replace the dramatic pause or a pause to indicate confusion.
Girl: Like, OMG! Gambit is soooo Hot!!!
Someone who thinks she's ridiculous but doesn't hate her enough to hurt her feelings: Dotdotdot.