The need for matzah shmurah comes from the Toraitic statement “U’shmartem Es HaMatzos” (“And you shall guard the Matzos”).There are three types of shmurah, which literally means “watched.”
As previously mentioned, if water comes into contact with one o the five species of grain and is allowed to stand for 18 minutes, it becomes chametz. If dough is constantly kneaded, it will not ferment. Matzah must be made with these five species of grain and also must be made with water. What is required is guarding that the dough or the grains not be allowed to stand 18 minutes once they have come in contact with water. They must be baked within that time.
The three types of shmurah matzah are:
Mishaas K’tzirah – watched from the time of reaping (harvesting)
Mishaas Techina – from the time of grinding (milling)
Mishaas Lisha – from the time of kneading
A grain may become chametz any time after reaping (after it is removed from its organic source of growth) if it has come into contact with water. Therefore it must be carefully guarded.
Although we eat matzah all Pesach, one is only legally obligated to eat matzah on the first night of Pesach (in Eretz Yisroel) and the first two nights in the diaspora. This of course does not mean that chametz may be eaten; IT MAY NOT. Therefore we are especially careful with the matzos that we eat that night (or those nights). Matzos commonly bought in supermarkets or grocery stores that are “Passover Matzos” are also shmurah matzos. However they are NOT sufficient to fulfill one’s obligation. First, they are only watched from the time of kneading, which is in itself not desirable, but more importantly, they lack another essential condition, that of being baked “L’Shame Matzas Mitzvah” (“for the sake of the commandment”).Therefore while they may be permitted the rest of Pesach they may not be used to fulfill one’s obligation to eat matzah on the first night of Pesach (first two in diaspora). Most G-d fearing people eat only matzah shmurah mishaas k’tzirah the entire Pesach. However, those who do not do so have authorities upon whom to rely.