ICHTHUS is an acronym for “Jesus Christ, God’s Son, Savior” which spells the word “fish” in ancient Greek. (More at the Wikipedia article on the subject.)
This acronym seems to have originated in the postbiblical Christian world. That is, you won’t find it in the Bible. You will, however, find that the Bible does testify that Jesus is the Christ, the Son of God, and Savior of the world” (Matthew 16:16; John 20:30-31; 1 John 4:14).
An acronym is not a bad way to remember things. Especially important things.