John Gill Commentary Deuteronomy 5:23

"And it came to pass, when ye heard the voice out of the midst of the darkness, while the mountain was burning with fire, that ye came near unto me, even all the heads of your tribes, and your elders;" — Deuteronomy 5:23 (ASV)

And it came to pass, when you heard the voice out of the midst
of the darkness
The thick darkness, where God was, and with which the mountain was covered, (Exodus 20:21)

for the mountain did burn with fire ;
which is a reason both why the Lord spoke out of the midst of the fire, the mountain on which he descended burning with it and also for his speaking out of the midst of darkness, because not only a thick cloud covered the mountain, but it was altogether on a smoke, which ascended as the smoke of a furnace, (Exodus 19:16Exodus 19:18)

that you come near unto me, even all the heads of your tribes and your
elders ;
or wise men, as the Targum of Jonathan; by which it appears, that not only the common people were frightened at what they heard and saw on Mount Sinai, but those of the first rank and eminence among them, who were the most famous for their authority and wisdom.