Albert Barnes Commentary Exodus 16:15

"And when the children of Israel saw it, they said one to another, What is it? For they knew not what it was. And Moses said unto them, It is the bread which Jehovah hath given you to eat." — Exodus 16:15 (ASV)

It is manna - “Man” or “man-hut,” that is, white manna, was the name under which the substance was known to the Egyptians and, therefore, to the Israelites. The manna of the Sinai Peninsula is the sweet juice of the Tarfa, a species of tamarisk. It exudes from the trunk and branches in hot weather and forms small, round, white grains.

In cold weather, it preserves its consistency; in hot weather, it melts rapidly. It is gathered either from the twigs of the tamarisk or from the fallen leaves underneath the tree. The color is a grayish-yellow.

It begins to exude in May and lasts about six weeks. According to Ehrenberg, it is produced by the puncture of an insect. It is abundant in rainy seasons; in many years, it ceases altogether. The whole quantity now produced in a single year does not exceed 600 or 700 pounds.

It is found in the district between Wady Gharandel (that is, Elim) and Sinai, in Wady Sheikh, and in some other parts of the peninsula. Therefore, when the Israelites saw the “small round thing,” they said at once, “This is manna,” but with an exclamation of surprise at finding it, not under the tamarisk tree, but on the open plain, in such immense quantities, and under circumstances so unlike what they could have expected. In fact, they did not really know what it was, only what it resembled.