This is the bread of affliction.

During the Exodus, our ancestors had to leave Egypt quickly so that Pharaoh wouldn't have time to change his mind again and re-enslave them. They left in such a hurry, they didn't have time for their bread dough to rise. By eating the matzah, we remind ourselves that we must take every opportunity for liberation, even if we don't feel ready. If we wait, that opportunity may vanish and never come again.

But there are other explanations:

This is the bread our ancestors ate in Egypt.

Our ancestors experienced inexplicable suffering at the hands of the Egyptians. Why then, is matzah the central symbol for this holiday. If someone asks "How bad was it really?" why do we point to the matzah and say, "That's how bad"? 

Matzah is bland and generally unappealing. In addition, it is a fairly impractical food. Flour and water will naturally rise when left alone. If the Egyptians gave the Israelites matzah to eat, they would have to make a conscious effort to do so. Further, the simple ingredients have very little nutritional value. It's not the food used to make a worker productive; it's the food used to subdue and placate a prisoner. 

This is the bread of humility.

Matzah is the most simple form of bread you can have. It only has two ingredients. It's simple in flavor. It hasn't even puffed up yet. This leavening is a symbol of an inflated ego. With this understanding, matzah teaches us to critically consider the time and attention we require. It teaches us to let go of the superficial, and prioritize both our own basic needs and those of the people around us.

haggadah Section: Motzi-Matzah