We eat marror-the bitter vegetableto remind us of the bitterness of slavery, and we eat matza to remember how quickly we left slavery behind. However, at the Seder night we reverse the chronological order, first we eat the matza and only then do we eat the marror. The standard explanation is that we follow the order they're presented in the Torah: with matza and marror you shall eat the paschal sacrifice - first matza is mentioned and then marror, and so we eat them in that order. Additionally, sometimes we need distance to understand how truly dreadful something was. Only after someone knows the meaning of freedom (matza), can they come to understand the full misery of slavery (marror).