That Fish Symbol
The fish symbol you see on cars doesn't mean the person is a big fan of fishing. It is a Christian symbol. The symbol might seem like a new modern thing, but the symbol is one of the oldest Christian symbols. The symbol is known is the Ichthys.
It's All Greek to Me
The word Ichthys comes from the Greek word ikhthýs. The word is an backronym. A Backronym is a phrase that is a source from a word. Ichthys means Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ or in English translates into "Jesus Christ, Son of God, Saviour."
Older than Ichthys
The fish symbol is actually older than the its name Ichthys. The symbol was used by early Christians to identify followers to each other. Christianity was an new religion and the people who observed Christianity where often persecuted. The fish was a way to communicated with each other. Fish are an important symbol in the bible. Jesus tells his Apostles that he will make them "Fishers of Men".