On the simplest level, the only difference between a loaf of bread and matza is that the bread is inflated and matza is flat. Matza is the food of a humble slave, who does not have the time to let the bread rise, and who eats foods that will leave him feeling full for hours afterward. For this reason, matza is also called "the bread of affliction”. Matza commemorates the bread of slavery that the Jewish people ate in Egypt, prepared in haste, without the luxury of time to let it rise.
On a deeper level, the fact that the Jewish people also ate matza, slave food, at the moment of their exodus from slavery indicates that they were powerless to save themselves. They were slaves up to the last moment, and only through Divine intervention did they go free.