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.