This video will discuss the account of the Mount of Transfiguration in Matthew 17 and provide contextualizing information to understand some parts of it. Beginning at verse 1
1 And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart,
2 And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light.
To begin with, this obeys the law of witnesses, which is important because messengers bearing Priesthood keys are likely being sent in fulfillment of the promise to Peter in the previous chapter that he would be given the keys of the kingdom. The mountain setting should remind us of other occasions where people have come into the presence of God such as Moses in the mount (Moses 1:1) and receiving the law (Exodus 24:12-18), Nephi being carried away into an exceedingly high mountain (1 Nephi 11:1) and other theophanies. The other place where these experiences often take place is the temple as in, for example, Isaiah 6:1. Overall, the scene in this section is very similar to that experienced by Joseph Smith and Olivery Cowdery in the Kirtland Temple when they received priesthood keys and, indeed, we might infer from Doctrine and Covenants 124:28-30 that this must take place in a temple when one is available, while when this is not available, a mountain is often seen as a fit alternative.
3 And, behold, there appeared unto them Moses and Elias talking with him.
Elijah (Elias is his name in Greek) was taken up into heaven by a chariot of fire. Moses, the Lord took unto himself as we understand from Alma 45:19. So he was also taken up into heaven rather than entering the promised land, possibly one of the more glamorous prophetic retirement options and generally more sought after than other popular options such as extreme old age and martyrdom. The key thing to realize though is that both were reserved for this ministry as well as their later ministry restoring keys in the Kirtland Temple.
4 Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias.
5 While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
The bright cloud is sometimes referred to as the kabod (see, e.g., https://www.mormoninterpreter.com/can-a-man-see-god-1-timothy-616-in-light-of-ancient-and-modern-revelation/) which protects human beings from the hazards of the immediate glory of God. Note that God bears testimony of his son, because it is through him that we are saved. As the JST on John 1:19, points out “And no man hath seen God at any time, except he hath borne record of the Son; for except it is through him no man can be saved.”
6 And when the disciples heard it, they fell on their face, and were sore afraid.
Falling on ones face is actually a rather frequent response to theophany. For example, in Genesis 17:3 Jacob fell on his face when the Lord appeared to him. Joshua 5:14 shows a similar case in which Joshua is receiving a divine messenger. Balaam does the same in Numbers 22:31 when he saw “the angel of the Lord standing in the way, and his sword drawn in his hand”. This also occurs in a number of temple settings in the scriptures. For example, we have the dedication of Solomon’s Temple in 2 Chronicles 7:3, where “they bowed themselves with their faces to the ground upon the pavement, and worshipped, and praised the Lord, saying, For he is good; for his mercy endureth for ever.” We also have a similar display during King Benjamin’s address in Mosiah 4:1 in which the people “had fallen to the earth, for the fear of the Lord had come upon them.”
7 And Jesus came and touched them, and said, Arise, and be not afraid.
8 And when they had lifted up their eyes, they saw no man, save Jesus only.
9 And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
Again as with the Temple, we are not always privileged to share openly sacred experiences. In the case of the apostles, they were given a specific charge not to disclose their experience until a particular event had occurred, Jesus’s resurrection. Now they didn’t quite understand the resurrection at that point so it seems likely that they initially thought Jesus was figuratively telling them to never relate the vision, but when he was resurrected it all made a good deal more sense and they shared what they had seen and heard with divine approval. In Doctrine and Covenants 63:64 we read, “Remember that that which cometh from above is sacred, and must be spoken with care, and by constraint of the Spirit; and in this there is no condemnation, and ye receive the Spirit through prayer; wherefore, without this there remaineth condemnation.” We might also point out that the apostles didn’t receive the Holy Ghost until after Jesus’s death and resurrection and so prior to this time they would not have the Spirit to guide them in knowing when might and might not be appropriate in the Lord’s sight for them to share their experience.