What does Mark 9:15 mean?

The story of the transfiguration is given in Peter, James, and John's point of view. We aren't told anything that the disciples didn't witness. And, unlike other passages where the disciples are confused about Jesus' teachings, we aren't given insight into Jesus' thinking (Mark 6:6), the situation (Mark 6:52), or even a flash-forward of future events (John 12:32–33) to explain the wider context. As the four come down from the mountain and interact with the crowd, the focus turns again to Jesus.

Before Jesus took Peter, James, and John to a high mountain to witness His transfiguration, they had already been surrounded by a crowd (Mark 8:34). Apparently, it grew during their absence. We don't know exactly where they are. Jesus had had one last altercation with the Pharisees in Galilee and left them, abandoning His public ministry in His home region (Mark 8:13). He and the disciples went through Bethsaida (Mark 8:22), on the northern-most coast of the Sea of Galilee, north to the villages around Caesarea Philippi.

We are told that Jesus took Peter, James, and John to a "high mountain" (Mark 9:2). The highest mountain around Caesarea Philippi is Mt. Hermon, which is even farther north. If that's the case, the four must have traveled toward Bethsaida where the Jewish scribes would have been, although Luke says that this happened "on the next day" after the transfiguration (Luke 9:37), so they wouldn't have gone far. Christian tradition says they were at Mt. Tabor, which is southwest of the Sea of Galilee, on the southern edge of Galilee. Another option is Mt. Meron, which is northwest of the Sea of Galilee and just a seven-hour hike from Capernaum.