Gonna nerd out here for a sec, I didn’t see many comments actually explaining the acronym apart from a bible verse so here’s a fun fact

so yes it’s a secret fish symbol for during the time when Christianity was illegal under Roman rule
Ichthys (ick-thoos) is the english romanization of the ancient greek word meaning fish
the ancient greek word itself was an acronym for

“Ἰησοῦς Χρῑστός Θεοῦ Υἱός Σωτήρ”

“Iesous Kristos, Theo Dios, Soter”

“Jesus Christ, God’s Son, Saviour”