Exodus 16

The Israelites come to the wilderness of Sin and they grumbled against Moses and Aaron: “Would that we had died by the hand of the Lord in the land of Egypt, when we sat by the meat pots and ate bread to the full, for you have brought us out into this wilderness to kill this whole assembly with hunger”.  The Lord hears their grumbling and decides to provide for them.  He does this to test them: "Then the Lord said to Moses, “Behold, I am about to rain bread from heaven for you, and the people shall go out and gather a day's portion every day, that I may test them, whether they will walk in my law or not".  There are a number of specific instructions they are to follow with regards to this food that they are to receive.  They're to go out and gather it each day and not keep any leftover.  On the 6th day, they are to gather twice as much.  Whatever is leftover is to be kept for the 7th day where they are to eat the leftovers.  However, they do not listen and some leave leftovers and it breeds worms.  On the 7th day, some still go out to gather but there is none.  Even in these simple tasks the Israelites cannot follow God's instruction.

Well, this food that is provided is called Manna.  It is what the Israelites eat for 40 years while in the wilderness.  Moses is commanded to keep a portion of it so that generations after can see what they ate.