Answer: The first place the word manna (Strong's Concordance #H4478) is mentioned in the Bible is in Exodus 16:15. It is used only fourteen times in the entire King James Old Testament and five times in the New Testament (John 6:31, 49, 58, Hebrews 9:4 and Revelation 2:17).
The word literally means, "What is it." Scripture describes it as being like "coriander seed, white. And the taste of it was like wafers with honey" (Exodus 16:31, HBFV).
Exodus 16:4 states that God rained bread from heaven in order to feed his people after they left Egyptian bondage. This miracle in the wilderness has no counterpart today. It was the staple food for a few million Israelites during their sojourn in the wilderness. This miraculous substance, however, was more than just food.
Manna was used as a teaching tool to convey an important lesson regarding obedience and to underscore the acceptable day to worship our Creator. During the generations of Egyptian slavery, the Israelites had lost track of God's Sabbath day. As later verses in chapter 16 of Exodus show, this daily meal became God’s teaching tool. It taught or revealed the correct day to keep the Sabbath and what was permissible on it (Exodus 16:22 - 30).
After the children of Israel received God's law and built a tabernacle in the wilderness, they were commanded to place some of the manna inside the Ark of the Covenant (Hebrews 9:4). It was hidden, in a sense, inside an earthen vessel (baked clay) overlaid with gold that was put in the ark.
Jesus, in Mark 2, revealed to the Pharisees what was permissible to do on the Sabbath by using the manna miraculously provided to the ancient Israelites as an example of how to properly observe the day (Mark 2:24 - 28).
The apostle John helps tie all this together by recording Jesus' words that link him to the miracle of food raining down from heaven. The Lord tells a group of Jews the following.
I am the bread of life. Your fathers ate manna in the desert, but they died. This is the bread which comes down from heaven so that anyone may eat of it and not die (John 6:49 - 51, HBFV).
Jesus is explaining that although the ancient Israelites ate the miraculous food, given them directly by God, they still died. He, however, as the Savior of mankind, is the "true bread" from heaven. If a person "eats" him (choses to adopt his way of life) they will not die.
A second question, related to your first one, is what is the "hidden manna" Jesus will give to faithful Christians after they are resurrected (Revelation 2:17). This verse states that Jesus will give some of this special substance to those who overcome.
Revelation's manna or food is symbolic of Jesus' life, ministry, and sacrifice that he freely offered to make possible the gift of eternal life. Where, however, is Jesus today? Is he not "hidden" in heaven until he returns to the earth? Christ, therefore, in Revelation 2:17 is referencing himself.
The one who has an ear, let him hear what the Spirit says to the churches. To the one who overcomes I will give the right to eat of the hidden manna . . . (Revelation 2:17, HBFV).