Exode 16:4

And the Lord said to Mosheh, Behold, I will cause the bread which hath been laid up for you from the beginning to descend from heaven: and the people shall go out and gather the matter of a day by the day, that I may try them whether they will keep the commandments of My law or not.

Behold, I will make bread rain. That is, the bread would fall like rain from heaven. The word “bread” (lechem) is used here in a loose sense meaning “food.” Elsewhere the term lechem is applied to meat, as in the phrase, lechem isheh—“it is the food of the fire-offering” (Vayikra 3:11), and fruit, as in the phrase, “Let us destroy the tree with its fruit (belachmo)” (Yirmiyahu 11:19).

That I may test them. The test was that they would need His assistance every day (Ibn Ezra).

BEHOLD, I WILL ‘MAMTIR’ (CAUSE TO RAIN) BREAD FROM HEAVEN FOR YOU. Rabbi Abraham ibn Ezra wrote that because the manna came down like rain from heaven, He said mamtir, [which is derived from the root matar (rain)]. But we find: ‘yamteir’ (He will cause to rain) coals, fire and brimstone; And the Eternal ‘himtir’ (caused to rain) upon Sodom and upon Gomorrah brimstone and fire. [Brimstone and fire do not come down like rain, and yet Scripture uses the word matar with reference to them!] Perhaps in these cases they accompanied the rain of which the term “coming down” may properly be used. Onkelos’ opinion is that mamtir just means “cause to bring down,” for he translated: “behold, I will cause to bring down.” [However, it has no connection here with rain, as Ibn Ezra would have it.]
It is possible that the word mamtir can be associated with the expression ‘kamatarah’ (as a mark) for the arrow, although they are of different roots. (target) is of the root natar (keep), since a target is kept in sight and watched. Mamtir however is of the root matar (rain). Yet, as Ramban concludes, they have a common association, as is explained in the text. Scripture makes use of both terms when speaking of every form of “falling from above.” Thus it is called matarah (target) because they come down like arrows [on a target], and it says, ‘vayamteir’ (And He caused to rain) upon them flesh as the dust, and winged fowl as the sand of the seas. It may be that [Scripture does not use the term mamtir for every form of “falling from above,” but only] for the fowl of heaven. [Therefore, the term, vayamteir in the above — mentioned verse refers only to the winged fowl] because they came down upon them as the rain.

ויצא העם ולקטו דבר יום ביומו, even if it had been a man’s intention to collect what he perceived to be a large quantity (far more than the appropriate amount) it would turn out to be no more than was required for each day. This is what is described in verse 18, that when they measured it after returning to their respective tents they found that each person had returned to his tent with the daily ration of an omer multiplied by the number of people in his immediate family.

למען אנסנו, the trial consisted in the fact that the people had to look to G’d on a daily basis for their immediate food supplies. This experience would bring home to them that on the one hand they could rely on G’d, and on the other hand, as stated in Deuteronomy 8,3 such a lesson was only learned when, even briefly, for a day, they had been “starved” by G’d.

והכינו את אשר יביאו והיה משנה, not literally: “bread,” but מזון, “food.”

למען אנסנו הילך בתורתי, if when receiving their food without effort they will take care to keep My commandments, seeing they will then not have any excuses not to. This idea has been formulated in the classic statement by our sages in the Mechilta, section ויסע chapter 2 “the Torah, as an instrument of profound study, has been given only to the generation who ate the manna.” [no other generation had so little to distract it from devoting time to Torah study. Ed.]