Matthew 17.5

A bright cloud overshadowed them, “overshadow” here means, rather, to “be diffused” or “spread” over them.
It does not mean that it made a shade.
A cloud was the symbol of the divine presence.
Thus, God went before the Israelites in a cloudy pillar - dark by day and bright by night Exodus 14:19-20;.
He appeared on Mount Sinai in a cloud bright by fire

Exodus 24:15-17; and a cloud, the symbol of the divine presence - called the Shechinah - dwelt continually in the most holy place in the temple,
1 Kings 8:10-11;
Ezekiel 1:4; Ezekiel 10:4.
When, therefore, the disciples saw this cloud, they were prepared to hear the word of the Lord.