What does the fish mean? 
The fish is an ancient Christian symbol known from the 1st century catacombs in Rome. The first literary reference to a fish as a Christian symbol is from Clement of Alexandria (born c. 150) in Paedogogus, III, xi. During times of persecution, early Christians would scratch a fish symbol on the ground as a means to distinguish friend from foe.
 
The symbol itself may have come from the miraculous story of the loaves and fishes (John 6:1-13) or from the meal of fish Jesus shared with His disciples at the Sea of Galilee after the Resurrection (John 21:1-13).
 
Some prefer, however, the interpretation that it came from the story of the coin found by Simon Peter in the mouth of the fish (Matthew 17:24-27). The story may be seen as a metaphor for the free gift given to us by Jesus through his atoning death - our salvation.

The Greek word within the fish is ICHTHUS which means "fish" in Greek. It's also an acronym for the phrase Jesus Christ, Son of God, Savior.
 
Today, it’s origin and meaning are clouded by a vague tradition.  But the antiquity associated with this symbol was enough to make it accepted without question by modern  Christians who are striving for a closer identity with their roots.
Fish play a major roll in the gospels. 

For example:
Mark 1:17: "Come after Me, and I will make you become fishers of men."
Matthew 12:40: "...Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth."
Matthew 14:17: "And they said to Him, 'We have here only five loaves and two fish.'"
Luke 5:6: "And when they had done this, they caught a great number of fish, and their net was breaking."
Luke 24:42: "So they gave Him a piece of a broiled fish and some honeycomb."
John 21:6: "And He said to them, 'Cast the net on the right side of the boat, and you will find some.'
So they cast, and now they were not able to draw it in because of the multitude of fish."
1 Corinthians 15:39: "All flesh is not the same flesh, but there is one kind of flesh of men, another flesh of beasts, another of fish, and another of birds."
Ever wonder where the Christian symbol of the Fish came from? The Fish was the mark of the early Christian. The first believers didn't dare wear any mark of their faith. If they did so they were often persecuted or killed.

The Greek initials of "JESUS CHRIST, GOD'S SON, SAVIOR" made up the Greek word for Fish.

The early Christians sketched the sign of the fish as a sign to others that they were Christians.

So display your fish with pride, whether it's on your shirt, or on your car as a symbol to all that you are a Christian, and that JESUS CHRIST, GOD'S SON, IS YOUR SAVIOR.