Chapter 12 is a psalm describing what the people of Judah will experience when the Lord delivers them from their suffering.
The people will recognize and admit that they have sinned and have deserved to be punished. Yet, God does not stay angry with them. God replaces anger with consolation. The people have been afraid, but they will replace their fear with trust in God.
When they recognize the comfort that God offers them and they are able to trust, then they will be able to express their gratitude to God. Isaiah calls on them to tell others about the deeds that God has accomplished for them--and that God is still with them, living in their midst.
In his commentary, Isaiah 1-39, Walter Brueggemann says:
Chapters 1-12 have uttered Jerusalem to its sorry judgment and have imagined Israel in the nadir of its existnece, due to its recalcitrance. None of this is here denied. But if these chapters constitute an intentional unit, then it is important that israel's final word is praise and thanks. That is because Yahweh's final act is not wrath but comfort. Yahweh does indeed do harsh work. In the end, however, Yahweh will do otherwise. Yahweh will give an abundant life. Of this, Isael must sing. To this, all the others are invited as well.
We Christians are reading this oracle by Isaiah during our own time of disruption, fully recognizing how much of the pain we experience is the result of our own sinning. And we can continue to rely on the nature of God, a God who will bring comfort to our lives. Let us also respond to the gifts that God has given us by singing praise.