The "this" that Isaiah is referencing at this point is the decreed destruction of Jerusalem. God has announced it in terms of certainty and finality. And it will happen. Isaiah's point is that in judgment, God knows what He is doing.
The prophet uses an extended farming metaphor to get this across. Just as a farmer knows the right way to plow, and the right crops to plant in the right places; he knows the correct method of harvest and how to handle the crop after the harvest, so God know with infinite wisdom the best way to "thresh" Jerusalem. Isaiah is preparing the nation for the horrors of the siege and destruction of the city. He is assuring them that God will wisely use this to bring about His own purposes.
The bottom line is that the God Who brings judgment is always wonderful in counsel and excellent in wisdom. He can be trusted. The people can be reassured that even the coming devastation will be able to result in the best purposes of God. It is His time of threshing.