What does it mean for the Spirit to only speak of what he hears? (trinitarian perspective) I have much more to say to you, more than you can now bear. But when he, the Spirit of truth, comes, he will guide you into all the truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come. (John 16:12-13, NIV)


Am I correct that the Spirit of Truth is referring to the Holy Spirit?

If so, what does it mean for the Spirit to speak only of what he hears?

Trinitarians believe that the Holy Spirit is God, and God does whatever he wills to do. So how do they interpret these verses?
From John Gill's exposition of this passage:


  For he shall not speak of himself:
  as Christ, the Son, spoke not of himself in opposition to the Father, so the Spirit speaks not of himself in opposition either to the Father, or the Son, but in perfect agreement with both; being, as of the same nature and essence, power and glory, so of the same mind, understanding, and will; and as they agreed and wrought jointly and harmoniously, in the works of nature and providence, so in the economy of grace and salvation.
  
  But whatsoever he shall hear, that shall he speak;
  as Christ himself did, ( John 15:15 ) ; and they are such things as ear has not heard besides; what were secretly transacted in the council and covenant of peace, and agreed upon by all the three persons; things which concern the salvation of men, the Gospel church state, another world, and the glory of all the divine persons: ... (source)


This is extremely useful in two respects:


Recognising the Spirit when He comes: What the Spirit says will align with what the Father says and what the Son says - His voice will be familiar in that sense and a previous familiarity with words from the Father and the Son will serve as a suitable 'introduction' for a believer to therefore recognize the voice of the Spirit when He speaks.
A Safeguard from error - particularly Montanist style errors. There are many who claim to have received the Spirit, but are actually deceived as the words they claim as being from the Spirit are inconsistent with words that come from the Father and the Son. This passage of scripture makes it abundantly clear that the Spirit will not contradict the revelation previously given, therefore prophecies, 'words of wisdom' or 'words of knowledge' should be subjected to the test of whether they are consistent with scripture.