Let's look at these sentences: "Oh, go on!" "Sit down, please" "Keep quiet!" "Open the door!" "Come here, baby!" Could you see the similarities between these sentences? There are only stand-alone phrases, like "go on", "come here" "keep quiet", but without any subject like "you", "He" etc in them. These kind of sentences are named imperative sentences, and are applied to express encouragement, order, warning etc.