"Men will flee to caves in the rocks and to holes in the ground from dread of the Lord and the splendor of his majesty, when he rises to shake the earth." NIV translation
Isaiah is telling the people of the day that Jesus returns to the earth. Those that denied him will now see the power and splendor of the Lord. It will no longer be possible to deny the truth of God and so they will be terrified as they realize that judgment has come. John says, in Revelation, that they will be so terrified that they will beg to die.
"In that day men will throw away to the rodents and bats their idols of silver and idols of gold, which they made to worship." NIV translation
Isaiah goes on to explain that on that glorious day of the coming of the Lord, all the idols will be cast down. Mankind will realize that all of their machines, buildings, etc. will mean nothing when the Creator of the universe shows up. Nothing can hide or protect them from the coming judgment.
"They will flee to caverns in the rocks and to the overhanging crags from dread of the Lord and the splendor of his majesty, when he rises to shake the earth. Stop trusting in man, who has but a breath in his nostrils. Of what account is he?" NIV translation
Isaiah repeats that, on the judgment day, the people will attempt to flee from the Lord. He is almost begging the people to stop trusting in men and put their faith in God. He concludes with a question to try to get them to see that it does not make sense to trust in men and the things of their hands. This same plea should be ours as Christians as we try to show others the hope that we have in Jesus Christ. But first, we must examine ourselves to make sure that we have put God first in our lives (anything less is the same kind of idolatry that Isaiah was speaking of).