The fashion - The "appearance."
Glistering - Shining like lightning - of a bright, dazzling whiteness. As Mark says, "more white than any fuller could make it."

And as he prayed, the fashion of his countenance was altered,.... It became exceeding bright and glorious, it shone like the sun, Matthew 17:2 and hereby his prayer was answered; and thus, as Christ was heard and answered, whilst he was yet speaking, so are his people sometimes; and even their countenance is altered, when they are favoured with communion with God, and instead of a sad and dejected countenance, they have a cheerful one.
And his raiment was white and glistering; it was as white as the light, as snow, and whiter than any fuller on earth could whiten, as the other evangelists say, and so glistened exceedingly; See Gill on Matthew 17:2. See Gill on Mark 9:3.

as he prayed, the fashion, &c.--Before He cried He was answered, and while He was yet speaking He was heard. Blessed interruption to prayer this! Thanks to God, transfiguring manifestations are not quite strangers here. Ofttimes in the deepest depths, out of groanings which cannot be uttered, God's dear children are suddenly transported to a kind of heaven upon earth, and their soul is made as the chariots of Amminadab. Their prayers fetch down such light, strength, holy gladness, as make their face to shine, putting a kind of celestial radiance upon it (2-Corinthians 3:18, with Exodus 34:29-35).
raiment white, &c.--Matthew says, "His face did shine as the sun" (Matthew 17:2), and Mark says (Mark 9:3), "His raiment became shining, exceeding white as snow, so as no fuller on earth can white them" (Mark 9:3). The light, then, it would seem, shone not upon Him from without, but out of Him from within; He was all irradiated, was in one blaze of celestial glory. What a contrast to that "visage more marred than men, and His form than the sons of men!" (Isaiah 52:14).