The term amounts to a raised finger underscoring the significance of the next words. John himself as first hearer of these words—and eventually (the members of) the seven churches who hear this Revelation being read to them (Revelation 1:3, Revelation 1:11)—needs to pay particular attention to what is about to be said.
12 “Behold, I am coming soon, bringing my recompense with me, to repay each one for what he has done.