The Sages prohibited kneading dough for Pesaḥ matzot with lukewarm water, as warmth accelerates the leavening process. Unless workers are especially quick to knead and bake it, there is concern that it will become ḥametz. The Sages forbade kneading the dough even with regular cold water found in cisterns and in springs, lest the surrounding ground, which had absorbed the heat of the sun, in turn warmed the water. Therefore, they required that the water be drawn before nightfall and kept overnight in a cool place. Such water is called mayim she-lanu. This water is used to prepare matzot for Pesaḥ (SA 495:1 and 3).
However, a problem arose in countries with warm climates. No matter where the water was kept overnight, it would warm up a bit. On the contrary, had the water been left in the springs, it would have stayed cooler! Nevertheless, the halakha is that the water must be left out, as the Sages enacted. If, as a result, the water warms up a bit, it must be refrigerated (Mikra’ei Kodesh 2:7).
Some maintain that one should leave tap water overnight for baking the matzot, since the water might be from an open reservoir and may have been warmed by the sun. Furthermore, there is concern that the chlorine mixed into the water will accelerate the leavening process (She’arim Metzuyanim Be-halakha 109:3). In practice, this possibility is of no concern, and only a few meticulous people who make handmade matzot are scrupulous about drawing the water from wells or springs. In machine-matza factories, they take regular tap water from the municipal system, filter it thoroughly, and leave it in a cool place all night (R. Tzvi Yehuda Kook similarly would prepare mayim she-lanu from tap water).