With that word the speaker demands the attention of his hearer for what he is going to say next. The implication is that his interjection is of such importance that one needs to pay careful attention. The hearer, of course, is the audience John has been addressing: the seven churches, mostly people downstream from the kings mentioned in Revelation 16:14 and overwhelmingly victims to whatever decisions kings make.
15 (“Behold, I am coming like a thief! Blessed is the one who stays awake, keeping his garments on, that he may not go about naked and be seen exposed!”)