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 Old Testament and five times in the New Testament. The word literally means, "what is it." The Bible describes it as being like "coriander seed, white. And the taste of it was like wafers with honey" (Exodus 16:31, HBFV)
The book of Exodus verse 4 of chapter 16 states that God rained bread or manna 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.
During the generations of Egyptian slavery, the Israelites had lost track of God's Sabbath day. As later verses in this chapter 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.
In Mark 2 Jesus revealed to the Pharisees what was permissible to do on the Sabbath day, just like the bread from heaven taught the ancient Israelites 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. In chapter 6 verses 49 to 51, Christ states that although the children of Israel ate this miraculous food in the wilderness they died. He, however, as the Savior of mankind, is the true bread from heaven that if anyone eats they will not die.
Your second question centers on what the 'hidden manna' is that Revelation discusses in chapter 2 verse 17. This verse states that Jesus will give some of this special substance to those who overcome. This food is symbolic of his 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, is referencing himself.