being a secular Jew in Israel, i've never given it any thought.
it's tradition, and you just go through the usual routine without thinking about it too much.
i'm sure i've learned something about it in kindergarden or 1st grade or whatnot, but who remembers ?
the Parsley you referred to is called "Maror" in the Haggadah. the word "Maror" is derived from "Mar" which means "bitter" in Hebrew. or maybe it's the Horseradish that is the Maror? i'm really not sure. was it the red horseradish that is called "Chazeret"?
even here in Israel where Hebrew is spoken, i don't know what Maror is supposed to be. i mean, there's no such vegetable at the green-grocer. it's "represented" by some other kind of green, it varies, different people use a different vegetable for it.
yesterday instead of parsley in saltwater i had a turnip in saltwater.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the land of Mordor, where the Shadows lie
M.C. Escher - Reptilien