It's a fish! But more than a fish! To understand why the fish is a christian symbol you gotta understand a little bit of the greek. Basically the letters for fish in greek: ἸΧΘΥΣ (or IKhThUS), is an acronym for 'Jesus Christ, Son of God, Savior'.
Iota (i), Iēsoûs (Ἰησοῦς), "Jesus"
Chi (ch), Khrīstós (Χρῑστός), "anointed"
Theta (th), Theoû (Θεοῦ), "of God", the genitive singular of Θεóς, Theós, "God"
Upsilon (y or u), (h)uiós[10] (Yἱός), "Son"
Sigma (s), sōtḗr (Σωτήρ), "Savior"
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