There is a Torah commandment to eat matza on the night of the fifteenth of Nisan, as it states: “In the evening, you shall eat matzot” (Shemot 12:18). This matza must have been guarded (shmura), as it states: “And you shall observe (u-shemartem) the matzot” (ibid. 17), and some poskim say it must be made by hand, with the specific intention of fulfilling this mitzva (see above 12:4). One who eats stolen matza does not fulfill his obligation (SA 454:4). Therefore, it is good to pay for the matza before Pesaĥ, or at least obtain the explicit consent of the storeowner to grant the buyer ownership of the matza even if it has not yet been paid for, because if the storeowner does not agree to give the matza on credit, one cannot fulfill his obligation with it (MB 454:15). As soon as one eats a kezayit of shmura matza he has fulfilled the Torah commandment, because all eating-related commandments in the Torah require the consumption of at least a kezayit.
Beyond the Torah obligation, the Sages ordained three more kezeytim of matza to be eaten at the Seder, making a total of four. After reciting the berakhot of “ha-motzi” and “al akhilat matza,” we eat, le-khatĥila, two kezeytim: one from the top matza, for “ha-motzi,” and one from the broken middle matza for “al akhilat matza.” Later, we eat another kezayit with maror, for korekh, and at the end of the meal we eat one more kezayit as the afikoman (some say it is preferable to eat two kezeytim for the afikoman).
Before getting into the specifics of the size of a kezayit, let us clarify the practical halakha: there is a consensus that a kezayit is about a third of a piece of machine-made matza, and about the same-sized piece of a handmade matza. Thus, right after reciting “ha-motzi” and “al akhilat matza,” two-thirds of a machine matza must be eaten. Another third should be eaten for korekh and one more for the afikoman (for those who are stringent about eating two kezeytim for the afikoman, one half of a matza suffices for this purpose).
The kezayit of matza must be eaten continuously. If one pauses while eating, and as a result takes longer than a shi’ur akhilat pras to eat a kezayit, he does not fulfill the mitzva. We shall soon discuss exactly how much time a shi’ur akhilat pras is, but for now, it is enough to say that whoever eats a kezayit of matza continuously fulfills the mitzva without question and need not look at the clock, because the only way it is possible to take longer than akhilat pras is if one stops eating for a few minutes.
. Some stringently insist that the head of the household purchases the matza specifically for his guests and adult children, so that they eat matza that belongs to them. See Sidur Pesaĥ Ke-hilkhato 2:8:3. Nevertheless, one who eats matza owned by the head of the household fulfills his obligation, even if he performed no act of acquisition, since one can fulfill his obligation with borrowed matza (MB 454:15).