"Using a bitter herb as part of the Passover table Seder ritual helps embody some aspect of the physical experiences and triggers our memories. Of course, no one would compare eating spicy or hot food with slavery, yet it helps create the conversation and the experience of discussing hard or difficult life situations. The ritual helps us engage in the deeper conversation of mindful living in community." Rabbi Becker-Klein, 

What to use for bitter herbs can be confusing:

Some Seder plates have a place for both  maror  (bitter herbs) and  chazeret  (a word that probably means “greens”). Some only have a space for  maror . The Mishnah is no help. It omits the word  maror  and says  chazeret  twice (m. Pesahim 10:3).

It is actually possible to use lettuce for all three ( karpas, maror, and chazeret ).

Maybe it will help to separate out all four of the rituals and the possible vegetables:

(1)  Karpas  is dipped in salt water. Could be parsley or lettuce (or other greens).

(2)  Maror  is dipped in  charoset  (the apple mixture). Could be slices of horseradish or lettuce or other greens. Can’t exactly do this with prepared horseradish unless you use a piece of matzah to hold the horseradish (that is what I have done in the past, but see below).

(3) The  chazeret  just kind sits there (if your Seder plate has a place for it). I usually say something about it (“the root of life is bitter . . . without redemption”). You could use a horseradish root, a piece of lettuce, or an onion (I use an onion or a green onion).

(4) The Hillel sandwich ( korech ) is matzah with some  maror  and  charoset  (it used to have lamb on it). If you use slices of horseradish root for the  maror , this part is weird. How do you put “half an egg’s volume” of sliced root on a matzah and eat it? You’d have to be a termite! David Arnow says there is a debate about whether you should use prepared horseradish or slices of the root. The Hillel sandwich only makes sense, it seems to me, with prepared horseradish.

haggadah Section: Maror