The early Christians spoke Greek and the Greek word for fish is "ichthys". As early as the first century, Christians used the individual Greek lettersfor the spelling of the word fish (ichthys) to create an acrostic or secret meaning Iesous Christos Theou Yios Soter, which translates to:
Jesus Christ Son of God, Savior.
Because they faced persecutions for their faith during the first century after Christ, they used the fish symbol as a password or secret symbol.
Since this symbol was widely used by Greeks, Romans and other pagans during that time with other meanings, it was a symbol that would not arouse much suspicion. The Christian fish symbol was used to mark tombs and secret meeting places.
It is said that when strangers met, the Christian would draw an arc on the ground and wait for the other to respond. If the other was a Christian, he would respond by drawing the lower arc to form the outline of a fish.