14 And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth. 15 John testified about Him and cried out, saying, “This was He of whom I said, ‘He who comes after me has a higher rank than I, for He existed before me.’” 16 For of His fullness we have all received, and grace upon grace. 17 For the Law was given through Moses; grace and truth were realized through Jesus Christ. 18 No one has seen God at any time; the only begotten God who is in the bosom of the Father, He has explained Him.
You may have encountered opening a chip (crisps, whatever) packet and found it was half full, and felt somewhat ripped off. Or maybe if you are a beer drinker, you've taken a sip only to find there is more froth than beer. Now who would want to go to a self-serviced petrol (gas for my US friends) station and asked 'filler up', only to stop half way down the road with an empty tank?
John writes that Jesus was full of grace and truth. Some may have some of the truth, some may reflect some of the divine grace, but only Jesus is full of it (so to speak). Paul in Colossians also says that in Christ, the fullness of God was pleased to dwell. We might say that Jesus is not half-assed about grace and truth, he goes the whole hog with it. In dying on a cross he embodies (and in a sense is disembodied by the Romans) the grace of God in dealing with human sin and evil - grace upon grace indeed. And because the Law through Moses divided Jew from non-Jew, this grace is freely available to all people.
Likewise, Jesus is embodied truth, the glory or image of God in all its fullness. In Exodus we often read of God's glory, and in 33:18 Moses asks to see it, but God responds that no one can see his face. Jesus, the only Son has seen God's face and makes God known. You can't know someone unless they choose to show you, else get to know someone who knows them better than anyone else. Ever had a friend tell you about someone important in their life so much that one you finally meet this person, you feel like you know them? Only Jesus can do this for us with regards to God.
Another aspect of glory is written about in Romans 3 - sin means falling short of God's glory, being less than what we were made for, less than human! In Christ, that image can be restored. We can shoot for glory, and even though our arrows may fall short, yet our aim is made more true by the one who shows us what the target is.