In 1500 BC, Israel was enslaved in the land of Egypt. Through Moses, God freed them, and there were about 10 million people at the time. After being freed from Egypt, Israel lived in the wilderness for 40 years before entering Canaan, so their biggest concern was food. In response, God rained down manna from heaven, and the Bible testifies to this.

[Exodus 16:14~31] When the dew was gone, thin flakes like frost on the ground appeared on the desert floor. When the Israelites saw it, they said to each other, "What is it?" For they did not know what it was. Moses said to them, "It is the bread the LORD has given you to eat....(31) The people of Israel called the bread manna. It was white like coriander seed and tasted like wafers made with honey. 

God sent down manna for Israel every day, and the people took it, baked it, steamed it, and sometimes ground it to make bread. So God put the manna in jars and placed them before the ark of the covenant in the holy of holies, as a sign that He had given Israel food to eat.

[Exodus 16:32~34] Moses said, "This is what the LORD has commanded: 'Take an omer of manna and keep it for the generations to come, so they can see the bread I gave you to eat in the desert when I brought you out of Egypt.' " So Moses said to Aaron, "Take a jar and put an omer of manna in it. Then place it before the LORD to be kept for the generations to come." As the LORD commanded Moses, Aaron put the manna in front of the Testimony, that it might be kept. 

But the earthly temple was made as a type and shadow of the heavenly temple. So what did the manna prophesy? Yehoshua mentions the manna to the Jews seeking a sign of Christ and tells them that he is the substance of the manna. 

[John 6:30~51] So they asked him, "What miraculous sign then will you give that we may see it and believe you? What will you do? Our forefathers ate the manna in the desert; as it is written: 'He gave them bread from heaven to eat.'" Jesus said to them, "I tell you the truth, it is not Moses who has given you the bread from heaven, but it is my Father who gives you the true bread from heaven. For the bread of God is he who comes down from heaven and gives life to the world....(48)I am the bread of life. Your forefathers ate the manna in the desert, yet they died. But here is the bread that comes down from heaven, which a man may eat and not die. I am the living bread that came down from heaven. If anyone eats of this bread, he will live forever. This bread is my flesh, which I will give for the life of the world." 

Just as the Israelites received life by eating manna in the wilderness, God's people can receive eternal life by eating the substance of the manna, the flesh and blood of Yehoshua, but not by actually eating and drinking it. Yehoshua compared his flesh and blood to food, and he was showing that just as food remains in the body of the person who eats it, so Yehoshua's flesh and blood must remain in people's bodies in order for them to receive life. Thus, this is a demonstration of the coming of the Holy Spirit.

[John 14:23] Jesus replied: If anyone loves me, they will obey me. Then my Father will love them, and we will come to them and live in them. 

[Colossians 1:26~27] the mystery that has been kept hidden for ages and generations, but is now disclosed to the saints. To them God has chosen to make known among the Gentiles the glorious riches of this mystery, which is Christ in you, the hope of glory. 

Therefore, the reality of manna refers to Yehoshua who came as the Holy Spirit. 

If you look at the temple on earth, there was a golden jar containing manna in the Most Holy. But this shows what Yehoshua, the reality of the heavenly temple, will accomplish. After His death on the cross as a sacrifice, Yehoshua entered the Most Holy through resurrection and ascension, fulfilling the prophecy that the high priest of the earthly temple would enter the Most Holy.

[Hebrews 9:11~12] Christ came as the high priest of the good things that are now here. He also went into a much better tent that wasn't made by humans and that doesn't belong to this world. Then Christ went once for all into the most holy place and freed us from sin forever. He did this by offering his own blood instead of the blood of goats and bulls. 

Then, through his resurrection and ascension, he entered the Holy of Holies and descended the Holy Spirit, so that those who received the Holy Spirit could eat the manna of life that rains down from heaven. So the manna stored in the Holy of Holies refers to the Holy Spirit of Jesus.