鱼哭了只有海知道我哭了只有心知道
[yú kū le zhĭ yŏu hăi zhī dào wŏ kū le zhĭ yŏu xīn zhī dào]
It compares the secrecy of crying like fish's tears in the sea to one’s own hidden sorrows only known by oneself. This implies personal pain and emotional isolation which others can't see.