What does John 18:2 mean?

Judas has already been identified as the traitor who will turn Jesus over to local religious leaders (John 13:2–3; Matthew 26:14–16). As with other details, John chooses to leave details of Jesus' last-minute prayers to the other Gospels (Matthew 26:36–46; Mark 14:32–42). He does the same with most of Judas' interaction with Christ's enemies (Mark 14:10).

Judas' participation is needed for two reasons. First, the priests and scribes want to apprehend Jesus when He's away from a supportive crowd (John 11:57). They need Judas to lead them directly to that spot. Second, in an era before photography, arresting soldiers would need someone to point out exactly which person was to be taken in. To avoid any possible confusion, Judas has arranged a sign to make it clear who Jesus is (Luke 22:47–48).

To this point, Jesus has been active in His role as Messiah, including His teaching and healing. When Judas arrives with priests and soldiers, Jesus will give one last demonstration of His power (John 18:4–6), proving that what happens is not outside His control. From that point forward, His path as Savior will involve allowing others to act on Him, instead.