When you eat the Passover meal, the bread that you eat must not have yeast in it. You must eat this kind of bread, which will be called the bread of suffering/misery, for seven days. This is to help you to remember all during the time that you are alive that when your ancestors left Egypt, where they were suffering because they were slaves, they left very quickly. They did not put in yeast and wait for the dough to swell up.