It melted - This refers to the manna which was not gathered.

And they gathered it every morning,.... From the time the morning began, during four hours, or to the fourth hour, as the Targum of Jonathan; that is, till ten o'clock in the morning: this was a very wonderful thing that bread should be provided and rained every morning about the camp of Israel, in such plenty as to be sufficient to feed such a vast body of people; and that for forty years together:

every man according to his eating; according to the number of persons he had in his family to eat of it, and as much as everyone could eat; there never was any lack or scarcity of it, but every morning there it was for them, and they gathered it, an omer for everyone:

and when the sun waxed hot, it melted; and so what was left ungathered, being exhaled by the sun and laid up in the clouds, generated and increased, and became a provision to be let down the next morning; it being the will of God that it should not be trampled upon or exposed; and that the Israelites might be diligent, and not neglect the time of gathering it: and the Targum of Jonathan says, from the fourth hour, or ten o'clock and onward, the sun was hot upon it and melted it, which is the time of day when the sun is hottest: and some things, we may observe, are hardened by the sun, and others are softened, as the manna was, even to a liquefaction; though otherwise it was of so hard a nature as to be beaten in a mortar and ground in a mill, Numbers 11:8.

And they gathered it every morning, every man according to his eating: and when the sun waxed hot, it melted.

16:13-21 At evening the quails came up, and the people caught with ease as many as they needed. The manna came down in dew. They called it Manna, Manhu, which means, What is this? It is a portion; it is that which our God has allotted us, and we will take it, and be thankful. It was pleasant food; it was wholesome food. The manna was rained from heaven; it appeared, when the dew was gone, as a small round thing, as small as the hoar frost, like coriander seed, in colour like pearls. The manna fell only six days in the week, and in double quantity on the sixth day; it bred worms and became offensive if kept more than one day, excepting on the sabbath. The people had never seen it before. It could be ground in a mill, or beaten in a mortar, and was then made into cakes and baked. It continued the forty years the Israelites were in the wilderness, wherever they went, and ceased when they arrived in Canaan. All this shows how different it was from any thing found before, or found now. They were to gather the manna every morning. We are hereby taught, 1. To be prudent and diligent in providing food for ourselves and our households; with quietness working, and eating our own bread, not the bread of idleness or deceit. God's bounty leaves room for man's duty; it did so even when manna was rained; they must not eat till they have gathered. 2. To be content with enough. Those that have most, have for themselves but food and raiment; those that have least, generally have these; so that he who gathers much has nothing over, and he who gathers little has no lack. There is not such a disproportion between one and another in the enjoyment of the things of this life, as in the mere possession of them. 3. To depend upon Providence: let them sleep quietly, though they have no bread in their tents, nor in all their camp, trusting that God, with the following day, would bring them in their daily bread. It was surer and safer in God's storehouse than their own, and would come thence sweeter and fresher. See here the folly of hoarding. The manna laid up by some, who thought themselves wiser, and better managers, than their neighbours, and who would provide lest it should fail next day, bred worms, and became good for nothing. That will prove to be most wasted, which is covetously and distrustfully spared. Such riches are corrupted, Jas 5:2,3. The same wisdom, power, and goodness that brought food daily from above for the Israelites in the wilderness, brings food yearly out of the earth in the constant course of nature, and gives us all things richly to enjoy.