During this part of the Seder, we create a sandwich made of Matzah, Maror and Charoset. 

This tradition comes from the description in Shemot that tells us to eat the Pascal Lamb Offering. Today as we do not sacrifice animals, we create this sandwich instead. We are supposed to eat starting from the bitter side and ending to the sweet, to remind us that slavery was bitter yet the outcome of freedom is sweet.

Hillel believed that the Matzah and Maror should be eaten together, while other sages believed that they should be eaten separately. This part of the Seder fixes that disagreement: they are eaten separately beforehand and then are together during Korech.

haggadah Section: Koreich