What does John 13:15 mean?

This statement is sandwiched between two references to Jesus' supreme authority. When Jesus approached Peter to wash his feet, Peter was scandalized (John 13:6–8). In that era, washing another's feet was an act of submission and humility. The idea that a master would do that to His own servant was unthinkable to someone in Peter's culture. At first, it would appear to contradict the idea of Christ as the Lord and Master.

Jesus clarifies that He's not denying His supremacy (John 13:13). Jesus fully accepts the idea that He is their Lord. That lordship is the point of what He did. The natural human perspective is to look at some types of service or love and say, "that's beneath me." This is especially true when we somehow feel more important or more valuable than others. By acting out humble, sacrificial, servanthood Jesus lowers the "beneath me" line all the way to the floor. Loving service to others is something no believer can brush off as unworthy of their time.

Here, Jesus also explicitly states that what He's done is meant to be an example. He does not mean that all Christians are obligated to literally, physically wash the feet of others. The implication goes far beyond a ritual. The point is that those who "follow" Christ ought to "follow" His example in humility and service.