What it means: The fish was a common symbol, and one of the earliest used by early Christians to identify themselves. It showed other Christians that one was a believer in Jesus and not a persecuter.
In the fish ( above ) are five Greek letters - IXOYE - "Iota, Chi, Theta, Upsilon, Sigma." These five Greek letters spell the Greek word for a fish, " Ichthus".

These five letters make an acrostic for the initials of the five Greek words that mean "Jesus Christ, Son of God, Savior."

I - Iota - Jesus ( Iesous ) "Jesus," is the Greek form of Joshua. Which in Hebrew means, "the Lord saves."

X - Chi - "Christ," ( Christos ) another term for the Messiah which means, "the Anointed one." The ancients made a special oil to anoint with. In the Old Testiment people were anointed to set them apart for service.

O - Theta ( Theou )- "God's"

Y - Upsilon ( Huios or Yios ) - "Son"
Son of God equivalent to the incarnate Son of God.

E - Sigma ( Soter )- "Savior," simply means one who saves. When Jesus made the Triumphal Entry to Jerusalem (Matthew 21:9 ) the crowds shouted "Hosanna" a Hebrew expression meaning "Saves."

Jesus saves us from sin, from evil and ultimately from death in that we shall be resurected.

Some of the disciples had been fishermen. Jesus spoke to them in terms they could relate to. "Come, follow me," Jesus said, and I will make you fishers of men." This is recorded in both the books of Mark, ( 1:17 ) and Matthew. ( 4:19 )