These passages are Matthew 17:1-9; Mark 9:2-9; and Luke 9:28-36.
Peter writes about it in 2 Peter 1:16-18.
Scriptures that raise a number of questions for debate, such as:
1) Did Jesus tell anyone in advance this would happen?
2) Why did it happen?
3) Were Moses and Elijah literally there?
4) Why did Moses and Elijah appear and then disappear?
5) Does what God said have any particular significance?
6) Why did Jesus say to tell no one about it until after his resurrection?
From HELPS Word-studies:
3339 metamorphó� (from 3326 /metá, "change after being with" and 3445 /morphó�, "changing form in keeping with inner reality") – properly, transformed after being with; transfigured.
[3339 (metamorphó�) is the root of the English terms "metamorphosis" and "metamorphize."]