What does Luke 9:34 mean?

Those who have been on a mountain on a sunny day, then watched as a cloud races in and envelops the landscape, have a small idea of what Peter, James, and John are experiencing. Even more so, however, they just woke up from a deep sleep to see their friend and teacher speaking with two historical icons. All three were shining with God's glory (Luke 9:28–32). And the cloud, itself, is the shekinah glory of God: a sight not seen in real life since Solomon dedicated the temple (2 Chronicles 7:1–3). Until now, such a display of God's glory was only seen in prophetic visions of the day of the Lord: the coming judgment of the world.

The Hebrew word shekinah isn't used in the Bible. Rather, it is found in extra-biblical rabbinic writings. The word means "he caused to dwell" and represents God's immediate presence among His people, specifically from within a cloud. Literal appearances include the pillars of cloud and fire that led the Israelites (Exodus 13:21) and the cloud on Mount Sinai (Exodus 24:15–18). Jewish literature further developed this as a reference to God's presence, a replacement for the metaphor of God's hand or face, and eventually a mediator between God and humans. John 1:14 echoes the concept of shekinah when referring to God's presence.

In other New Testament cases, when God speaks His glory doesn't come down; the skies open (Luke 3:21–22) or the people just hear His voice (John 12:28).

Luke states that "the cloud came and overshadowed them" and that the three disciples "entered the cloud" while Matthew and Mark only say the cloud "overshadowed" them (Matthew 17:5; Mark 9:7). There's no real contradiction; when you're in a cloud, you can still see around you somewhat even if the sky above is completely blocked.