Apostrophe is an exclamatory rhetorical figure of speech, when a speaker or writer breaks off and directs speech to an opponent or third party, to observers, or to an imaginary person, location, deity, or abstract quality or idea. This can be a person, a ghost, a God, or virtually any person that is not present. This allows the author to give information or explanations to a reader who would otherwise be difficult to know or understand. It takes some skill to be able to properly use the apostrophe figure of speech in a way that appears natural in the course of one's writing, but it has been done to great effect throughout history. The term is derived from a Greek word meaning "a turning away," and this sense is maintained when a narrative or dramatic thread is broken in order to digress by speaking directly to someone not there, e.g., "Envy, be silent and attend!", Alexander Pope. In dramatic works and poetry written in or translated into English, such a figure of speech is often introduced by the vocative exclamation "O". For example, in William Shakespeares' Julius Ceaser, Mark Antony addresses the corpse of Caesar in the speech that begins:
"O, pardon me, thou bleeding piece of earth,
That I am meek and gentle with these butchers!
Thou art the ruins of the noblest man
That ever lived in the tide of times."
Lines like "Then come, sweet death, and rid me of this grief." are also an example of apostrophe, in this passage, the speaker is talking to death which is, obviously, not a person, but a phenomenon.
Meaning: The poet addresses the sun in an informal and colloquial way as if it were a real human being. He asks the sun in a rudely that why he appeared and spoiled the good time he was having with his beloved.