The King James 21st Century Version has this; John 16:13 However when He, the Spirit of Truth, is come, He will guide you into all truth; for He shall not speak from Himself, but whatsoever He shall hear, that shall He speak; and He will show you things to come.What translations say he speaks "from himself" and what do you think "from himself" means?
The American Standard Version has the same John 16:13 Howbeit when he, the Spirit of truth, is come, he shall guide you into all the truth: for he shall not speak from himself; but what things soever he shall hear, these shall he speak: and he shall declare unto you the things that are to come.
Other Bible version in comparison with the KJV at this link ----> John 16:13 KJV & NIV & AMP & ESC & TLB in saying He cannot speak of Himself, He will not speak on His own, nor His own initiative, nor His own authority, nor presenting His own ideas.
As for what it means, the italics per the 2 examples above should answer that question as all Bible versions testify to this truth that the Holy Spirit cannot express Himself out loud anything from Himself, but speaks what He hears as God's gift of tongues is for speaking unto the people and not the Holy Spirit speaking back to God, not even in gibberish nonsense.