Some have argued that manna was natural exudates from Arabian trees and shrubs, but they are small in quantity and peculiar in taste, in contrast to the manna described in the Bible.
Manna still rains in the middle east today. The flavor of manna is like bread mixed with honeydew, and it is about the size of a corvadier seed. 
The Israelites had been complaining in the wilderness about there being no food. So the Majesty on High rained down Manna to feed them.
Then the Lord said to Moses, “Behold, I will rain bread from heaven for you. And the people shall go out and gather a certain quota every day, that I may test them, whether they will walk in My law or not. 5 And it shall be on the sixth day that they shall prepare what they bring in, and it shall be twice as much as they gather daily.” Exodus 16:4-5(NKJV).
The Israelites could only gather as much for one day in the morning, according to the family of one household. The manna would appear once the dew of the morning fell on the desert. Those who gathered more manna than for that day would find the next morning that the manna expired. This was to test the children of Israel.
- So He humbled you, allowed you to hunger, and fed you with manna which you did not know nor did your fathers know, that He might make you know that man shall not live by bread alone; but man lives by every word that proceeds from the mouth of the Lord. 
The only time the manna could be stored up for the next day was Friday, because of the Sabbath the next day.  23 Then he said to them, “This is what the Lord has said: ‘Tomorrow is a Sabbath rest, a holy Sabbath to the Lord. Bake what you will bake today, and boil what you will boil; and lay up for yourselves all that remains, to be kept until morning.’ ” 24 So they laid it up till morning, as Moses commanded; and it did not stink, nor were there any worms in it. Exodus 16:23-24 (NKJV).
Jesus, the Precious One of God, and the Manna from Heaven
30 Therefore they said to Him, “What sign will You perform then, that we may see it and believe You? What work will You do? 31 Our fathers ate the manna in the desert; as it is written, ‘He gave them bread from heaven to eat.’ ” 32 Then Jesus said to them, “Most assuredly, I say to you, Moses did not give you the bread from heaven, but My Father gives you the true bread from heaven. 33 For the bread of God is He who comes down from heaven and gives life to the world.” John 6:30-33 (NKJV) 
- Exodus 16:4-36
- Deuteronomy 8:3 (NKJV).