What is the great hail from heaven?
The great hail from heaven is the plague poured out in the seventh vial of God's wrath, when hailstones weighing about a talent (roughly 75 to 100 pounds each) fall upon the earth, and men respond by blaspheming God rather than repenting.
Revelation 16:17-21 describes the seventh angel pouring out his vial into the air, and a great voice from the throne declaring, It is done. This is followed by voices, thunders, lightnings, and the greatest earthquake the earth has ever known, so mighty that the great city is divided into three parts, the cities of the nations fall, every island flees away, and the mountains are no longer found. Then there falls upon men a great hail out of heaven, every stone about the weight of a talent, roughly 75 to 100 pounds each. Rather than turning to God in repentance, men blaspheme Him because the plague of the hail is so exceedingly great.
This hail is tied to other scriptures that speak of hail as an instrument of God's judgment and fury. Isaiah 28:17-18 speaks of hail sweeping away the refuge of lies, and Isaiah 30:30 describes the Lord causing His glorious voice to be heard with tempest and hailstones as a sign of His indignation and devouring anger. Ezekiel 13:10-15 also shows God sending great hailstones to break down false security, that is, the wall daubed with untempered mortar, so that people would know He is the Lord.
At the same time, there is comfort connected to this judgment. Isaiah 32:18-19 shows God's people dwelling in peaceable habitations and sure dwellings even when it hails, a picture of protection for His remnant. This is understood as an encouragement that, though that passage speaks of Israel's remnant, the sons of God can expect the same kind of divine protection when hail falls as part of God's wrath, since the Lord knows how to deliver the godly out of temptation while reserving judgment for the unjust, as stated in 2 Peter 2:9.
The scriptures
And the seventh angel poured out his vial into the air; and there came a great voice out of the temple of heaven, from the throne, saying, It is done. And there were voices, and thunders, and lightnings; and there was a great earthquake, such as was not since men were upon the earth, so mighty an earthquake, and so great. And the great city was divided into three parts, and the cities of the nations fell: and great Babylon came in remembrance before God, to give unto her the cup of the wine of the fierceness of his wrath. And every island fled away, and the mountains were not found. And there fell upon men a great hail out of heaven, every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great.
Judgment also will I lay to the line, and righteousness to the plummet: and the hail shall sweep away the refuge of lies, and the waters shall overflow the hiding place. And your covenant with death shall be disannulled, and your agreement with hell shall not stand; when the overflowing scourge shall pass through, then ye shall be trodden down by it.
And the LORD shall cause his glorious voice to be heard, and shall shew the lighting down of his arm, with the indignation of his anger, and with the flame of a devouring fire, with scattering, and tempest, and hailstones.
And my people shall dwell in a peaceable habitation, and in sure dwellings, and in quiet resting places; When it shall hail, coming down on the forest; and the city shall be low in a low place.
Because, even because they have seduced my people, saying, Peace; and there was no peace; and one built up a wall, and, lo, others daubed it with untempered morter: Say unto them which daub it with untempered morter, that it shall fall: there shall be an overflowing shower; and ye, O great hailstones, shall fall; and a stormy wind shall rend it. Lo, when the wall is fallen, shall it not be said unto you, Where is the daubing wherewith ye have daubed it? Therefore thus saith the Lord GOD; I will even rend it with a stormy wind in my fury; and there shall be an overflowing shower in mine anger, and great hailstones in my fury to consume it. So will I break down the wall that ye have daubed with untempered morter, and bring it down to the ground, so that the foundation thereof shall be discovered, and it shall fall, and ye shall be consumed in the midst thereof: and ye shall know that I am the LORD. Thus will I accomplish my wrath upon the wall, and upon them that have daubed it with untempered morter, and will say unto you, The wall is no more, neither they that daubed it;
The Lord knoweth how to deliver the godly out of temptations, and to reserve the unjust unto the day of judgment to be punished:
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.