Hashem (G-d) promised our Forefather Abraham that when the time came He would take us out of Egypt immediately. And indeed, when the time came, Hashem took us out of Egypt. The Egyptians, who until then had refused to let us go, suddenly were urging us and hurrying us away. We were sent out in such a hurry that we didn’t have time to let our dough rise to make bread. We wrapped up our dough, and baked it as soon as we were able. Since it had been so little time since we kneaded the flour with the water, the dough had not yet begun to rise. So we made what we call matzos, i.e., unleavened bread.
Thus the Torah says: “The Israelites traveled from Rameses toward Sukkoth. There were about 600,000 adult males on foot, besides the children…. They baked the dough that they had brought out of Egypt into unleavened (matzah) cakes, since it had not risen. They had been driven out of Egypt and could not delay, and they had not prepared any other provisions” (Exodus 12:37, 39).
Therefore, Hashem commanded us:
“This day must be one that you will remember. You must keep it as a festival to God for all generations. It is a law for all time that you must celebrate it. Eat matzahs for seven days. By the first day, you must have your homes cleared of all leaven. Whoever eats leaven from the first day until the seventh day will have his soul cut off from Israel…. Be careful regarding the matzahs, for on this very day I will have brought your masses out of Egypt. You must carefully keep this day for all generations; it is a law for all times. From the 14th day of the first month in the evening, until the night of the 21st day of the month, you must eat [only] matzahs. During [these] seven days, no leaven may be found in your homes. If someone eats anything leavened his soul shall be cut off from the community of Israel. [This is true] whether he is a proselyte or a person born into the nation. You must not eat anything leavened. In all the areas where you live, eat matzahs.”
(Exodus 12:14-20)
In addition, the Torah says:
“Eat matzos for seven days, and make the seventh day a festival to God. Since matzos must be eaten for [these] seven days, no leaven may be seen in your possession. No leaven may be seen in all your territories” (Exodus 13:6-7).
During the days of the Holiday of Passover we may not eat bread that has been allowed to rise, or bread that has been caused to rise. That is what “leaven” refers to.
The Hebrew word for “leaven,” or for “leavened” is “chometz.”
Without the addition of yeast, dough will become leavened, i.e, it will rise, if left alone for eighteen (18) minutes. For Pesach we make matzos from dough that contains only flour and water, and nothing else, and we roll out the matzos and bake them before 18 minutes have passed.
This is why we do not allow any water or any moisture to touch the grains or the flour at all, except when we are actually making the matzah. Then, of course, we pour specially prepared water onto the flour and knead it into dough, roll it into matzos, and bake it, all very quickly. This is done in a special Pesach matzah bakery. I have observed dough from the time water touches it until the time it is a finished matzah, in such a Pesach matzah bakery, and it took about five minutes from start to finish. No chance of it becoming chometz!
And since the Torah tells us to guard the matzos, we very carefully guard the matzos from the harvest until we eat them – to keep them away from any unacceptable moisture.
Another reason that we eat matzah on Passover is because when we were slaves the Egyptians fed us matzah instead of bread. Matzah takes longer to digest, and that way they felt they didn’t need to feed us as often.
Matzah was eaten by poor people. Rich people had slaves to do their work. Poor people did not have the time to let the dough rise, and had to bake it immediately. So they wound up eating matzah most of the time. This is one reason we refer to matzah as “Bread of the Poor.”