The beginning of the Gospel - called the prologue - gives the theme of the book: the eternal Son of God has become human in order to save the world. The first three verses contain one of the most powerful statements about Christ in Scripture: Jesus Christ - the Word - has always existed with the Father, He was involved in the creation of all things, and He is in fact God. This statement is so important that St. Ambrose of Milan joyously responds to it, "Let the soul that wishes to approach God raise itself from the body and cling always to that highest Good that is divine and lasts forever and that was from the beginning and that was with God, that is, the Word of God."
As profound and significant as all this is, there's another important fact: this eternal God did not merely stay in heaven to oversee the world, but He became flesh (1:14). Blessed Theophylact says "the Evangelist used the word flesh to show the boundless condescension of God, Who for our salvation assumed what was completely alien to His divine nature, out of ineffable, astonishing love for man." Christ came into the world He made - even though few recognized Him for Who He is and beheld His glory (1:10-12, 14) - and, being the only person Who has seen the Father, declared and explained the Father to humanity (1:18).
St. John explains what all this means for us. Jesus is "life" and "light" (1:4-5, 9), meaning, according to Blessed Theophylact, that He sustains all living things, gives spiritual life to humans, and enlightens the soul. Because Jesus became flesh, something remarkable has happened: "And of His fullness we have all received, and grace for grace" (1:16). We can be completely transformed - in fact, we can be deified (2 Peter 1:3, 4) - because Christ became human. St Athanasius the Great explains, "He was made man that we might be made god. He manifested Himself by a body that we might receive a conception of the unseen Father. He endured the hubris of humanity that we might inherit incorruptibility." By saying we have received "grace for grace," the apostle is telling us that through Christ we receive what the Orthodox Study Bible calls "an overabundance of grace" (at the same time, St. John Chrysostom reminds us, the grace we receive from Christ is "only a small portion of the whole" - we must never believe we are equal to God).
The chapter goes on to describe the ministry of St. John the Forerunner. St. John humbly but pointedly told the priests, Levites and Pharisees that he was not the Christ, the Prophet nor Elijah (1:20-21, 24); instead, St. Cyril of Alexandria teaches, the baptism he performed was "an introductory baptism, washing those defiled by sin with water for a beginning of repentance and teaching them to go up from the lower to the more perfect."
The chapter concludes with some of the disciples joining Jesus. Why should they follow Him, and what will they do? Jesus (and St. Philip) respond, "Come and see" (1:39, 46).