What does John 18:15 mean?

When Jesus was under threat from an arresting crowd, Peter demonstrated bravery—or at least rage—by lashing out with a sword (John 18:10). Fortunately for him, Jesus had given the soldiers plenty of reasons to ignore the disciples (John 18:6–9). When Jesus submitted to His arrest, the disciples ran away (Matthew 26:56). Peter circles around to follow at a distance (Matthew 26:58), along with another of the disciples. Especially since Jesus had made it clear He wanted the disciples to stay out of harm's way, this is not a smart choice.

This disciple is almost certainly John himself. Referring to himself in the third person is John's pattern (John 1:35; 13:23–25). His relationship to the high priest's family is not clear. Scholars suggest that John's mother (Matthew 20:20) was Salome (Mark 15:40; 16:1; John 19:25) who may have been the sister of Jesus' mother, Mary. Luke indicates that Mary was related to Elizabeth, the mother of John the Baptist, who was a son of the priest Zechariah (Luke 1:13, 36). This creates a line of family relations which might have connected John to the family of the priests. John is the only one to mention the name of the servant injured by Peter (John 18:10–11), adding weight to the idea that he was familiar with the high priest's household.

Scripture does not tell us exactly what that relationship was. However, John is known well enough that he's able to enter the courtyard in the middle of the night, and convince the doorkeeper to let Peter in (John 18:16).