What does John 13:2 mean?

"Supper," in this case, is a private meal between Jesus and His closest followers. Most likely, this was only the Twelve (Matthew 10:2), but Scripture never gives a list of those who are present. It's possible—but not certain—that other followers such as Jesus' mother or Mary Magdalene might have been watching these events. Details of this last supper are expanded in Matthew, Mark, and Luke. John chooses to provide details missing in those accounts, filling in his own eyewitness memories.

At this point, none of the disciples know that Judas is a traitor. Judas has already arranged with local religious leaders to arrest Jesus in private, in exchange for money. None of this is a surprise to Jesus, of course, who has always been submissive to the will and timetable of God the Father (Mark 14:36; John 7:6; 12:27). These first three verses, in fact, emphasize His knowledge of all that's happening.

The prior verse referred to Jesus' love, especially for believers. A potent expression of that love, shown here, is the grace Jesus shows toward someone who is not only a non-believer, but a betrayer and liar, as well. Judas is present when Jesus washes the feet of the disciples (John 13:5). He shares at least some of the food from this meal with Jesus (John 13:21, 26). When dismissing Judas to complete his betrayal (John 13:27), Jesus is subtle and kind (John 13:28–29). Even to a vile enemy, Jesus shows compassion.