When Jesus washed the disciples' feet, Peter did not know the purpose and point of washing the feet. Eventually, Peter proclaimed that Jesus will not wash Peter's feet!
In response to Peter's comment, Jesus told Peter that he would have no part if Jesus doesn't wash Peter's feet (13:8). Jesus's comment indicates that the feet washing has some significance. But then, Peter calls Jesus to wash not only the feet but also hands and head! However, Jesus told Peter that the one who just bathed only needs to wash feet because said person is completely clean (13:10). Though Jesus pointed out that not all of His disciples are clean because he knew from Judas's heart that Judas would betray him.
Jesus is making yet another analogy -- this time on feet washing vs body washing. Body washing is a reference to salvation (AKA being saved from the penalty of sin) upon believing in Jesus as Lord and Savior, which is a one time deal. However, the ones who bathed still have to wash their feet. Feet washing is a reference to repeating renewal in Jesus (AKA recovery from sin and refocus on Jesus) that is part of discipleship. Even though we may be able to go to heaven upon believing in Jesus, when we become disciples, we need to be renewed and refocus on Jesus multiple times should we lose track because such actions are how we can be a part of discipleship.
Speaking of discipleship, new believers can become disciples of Jesus by focusing their entire life on Jesus. Also known as committing your life to Jesus.