From: John 21:15-19
 When they had finished breakfast, Jesus said to Simon Peter, "Simon, son of John, do you love Me more than these?" He said to Him, "Yes, Lord; you know that I love You." He said to him, "Feed My lambs."  A second time He said to him, "Simon, son of John, do you love Me?" He said to Him, "Yes, Lord, you know I love You." He said to him, "Tend My sheep."  He said to him the third time, "Simon, son of John, do you love Me?" Peter was grieved because He said to him the third time, "Do you love Me?" And he said to Him, "Lord, You know everything; You know that I love You." Jesus said to him, "Feed My sheep.  Truly, truly I say to you, when you were young, you girded yourself and walked where you would; but when you are old, you will stretch out your hands, and another will gird you and carry you where you do not wish to go."  (This He said to show by what death he was to glorify God.) And after this He said to him, "Follow Me."
15-17. Jesus Christ had promised Peter that he would be the primate of the Church (cf. Matthew 16:16-19 and note on the same). Despite his three denials during our Lord's passion, Christ now confers on him the primacy He promised.
"Jesus questions Peter, three times, as if to give him a triple chance to atone for his triple denial. Peter has learned his lesson from the bitter experience of his wretchedness. Aware of his weakness, he is deeply convinced that rash claims are pointless. Instead he puts everything in Christ's hands. `Lord, You know well that I love You" ([St] J. Escriva, "Friends of God", 267). The primacy was given to Peter directly and immediately. So the Church has always understood--and so Vatican I defined: "We therefore teach and declare that, according to the testimony of the Gospel, the primacy of jurisdiction over the universal Church of God was immediately and directly promised and given to Blessed Peter the Apostle by Christ our Lord. [...] And it was upon Simon Peter alone that Jesus after His resurrection bestowed the jurisdiction of chief pastor and ruler over all His fold in the words: "Feed My lambs; feed My sheep" ("Pastor Aeternus", Chapter 1).
The primacy is a grace conferred on Peter and his successors, the popes; it is one of the basic elements in the Church, designed to guard and protect its unity: "In order that the episcopate also might be one and undivided, and that [...] the multitude of the faithful might be kept secure in the oneness of faith and communion, He set Blessed Peter over the rest of the Apostles, and fixed in him the abiding principle of this twofold unity, and its visible foundation" ("Pastor Aeternus, Dz-Sch 3051"; cf. Vatican II, "Lumen Gentium", 18). Therefore, the primacy of Peter is perpetuated in each of his successors: this is something which Christ disposed; it is not based on human legislation or custom.
By virtue of the primacy, Peter, and each of his successors, is the shepherd of the whole Church and vicar of Christ on earth, because he exercises vicariously Christ's own authority. Love for the Pope, whom St. Catherine of Siena used to call "the sweet Christ on earth", should express itself in prayer, sacrifice and obedience.
18-19. According to Tradition, St. Peter followed his Master to the point of dying by crucifixion, head downwards, "Peter and Paul suffered martyrdom in Rome during Nero's persecution of Christians, which took place between the years 64 and 68. St. Clement, the successor of the same Peter in the See of the Church of Rome, recalls this when, writing to the Corinthians, he puts before them `the generous example of these two athletes': `due to jealousy and envy, those who were the principal and holiest columns suffered persecution and fought the fight unto death'" ([Pope] Paul VI, "Petrum Et Paulum").
"Follow Me!": these words would have reminded the Apostle of the first call he received (cf. Matthew 4:19) and of the fact that Christ requires of His disciples complete self-surrender: "If any man would come after Me, let him deny himself and take up the Cross daily and follow Me" (Luke 9:23). St. Peter himself, in one of his letters, also testifies to the Cross being something all Christians must carry: "For to this you have been called, because Christ also suffered for you, leaving you an example, that you should follow in His steps" (1 Peter 2:21).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.