What does 2 Peter 1:18 mean?

This verse concludes the sentence begun in the previous verse. Peter, James, and John were given the privilege of seeing Jesus transformed from his earthly appearance as a man. While they watched, He was "transfigured" into his eternal, glorious appearance as the Son of God (Luke 9:28–36). As Peter writes in this verse, they also heard the very voice of God speaking from heaven, affirming Jesus as His Son.

For Peter, this is the most powerful confirmation of Jesus' message. Peter heard Jesus' teachings. He was there for most of Jesus' earthly miracles. So, when Peter speaks of being an "eyewitness," who is not making up stories (2 Peter 1:16), there are more things on his mind than this one event. However, the transfiguration was such a powerful experience that Peter sees it as the primary proof of all Jesus claimed to be.

Why were those three disciples allowed to see and hear such an amazing confirmation of Jesus' deity? Why were they there with Him on the holy mountain? Because Jesus wanted them to tell everyone about it after He was resurrected from the dead (Matthew 17:9). Peter continued to fulfill that mission right up until his final days on earth.