On the first day, on the 14th day of the month in the evening, you shall eat matzah, through the 21st day of the month in the evening. (Exodus 12:18)
The Jewish day starts at nightfall (as per the Creation narrative, e.g., “and it was evening and it was morning: the sixth day” – Genesis 1:31). So, Passover starts on the 15th day of the month of Nisan, which is nightfall on the 14th of Nisan. (Got it? Good.) It is a positive commandment to eat matzah on this evening whether or not there is a Temple and a Passover sacrifice.
Like the other Passover-related mitzvos, eating matzah reminds us of the miracles of the Exodus. Specifically, it alludes to how the Jews were rushed out of Egypt so that their dough had no time to rise and baked as matzah.
Eating matzah the rest of Passover is optional. Eating chometz – leavened bread – is forbidden, so matzah is recommended in its place, but one only MUST eat matzah on the first night. (See Mechilta on parshas Bo.) Outside of Israel, our practice is to observe two days of Yom Tov and to hold a Seder on the first two nights, so of course we eat matzah at both of them.
It’s interesting to note that chometz is forbidden on Pesach, while matzah is required. And yet, the difference between chometz and matzah is miniscule! They both have the same ingredients but for matzah, the baking must be finished within 18 minutes of the time the water touches the flour. One second longer and it becomes chometz. This relationship can be seen in the very words “chometz” and “matzah.” Each word has the Hebrew letters mem and tzadi. The only difference is that matzah has a letter hei and chometz has a ches. And the difference between a hei and a ches is extending one line just the teeniest bit further!
The details of this mitzvah include all the specifics about making matzah, such as the time and ingredients permitted, as well as the quantity of matzah one must eat to fulfill his obligation. These details are discussed in the first four chapters of the Talmudic tractate of Pesachim. In the Shulchan Aruch, they can be found in Orach Chaim 471 and in the Mishneh Torah in Hilchos Chometz u’Matzah chapter 2. This mitzvah is #158 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #23 of the 77 positive mitzvos that can be fulfilled today in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.