What does “I am the root and the offspring of David” mean in Revelation 22:16?

This verse has been claimed by some Trinitarians, Oneness and Arians as meaning that Jesus existed before David.

Revelation 22:16 is an allusion to Isaiah 11:1 and uses the same Greek word for root as used in relation to Jesse.

Isaiah 11:1 And there shall come forth a rod out of the stem of Jesse, and a Branch shall grow out of his roots:

Isaiah 11:1  καὶ ἐξελεύσεται ῥάβδος ἐκ τῆς ῥίζης Ιεσσαι καὶ ἄνθος ἐκ τῆς ῥίζης ἀναβήσεται 

The context of this verse is reestablishing the line of Davidic kings, but not through the line of Solomon, which had been cursed, but through Mary.

So the reference is to the promised offspring or descendant from Eve (Genesis 3:15), and from Abraham (Galatians 3:16), and David (2 Samuel 7:12).