What is the "essential act" of war in 1984?
In Part 2, Chapter 9, Winston is reading Goldstein's book and finding out why, exactly, the Party runs things the way it does. One aspect of the Party's control is war. This is where the phrase you are asking about comes in. Goldstein says that the destruction is the essential act (the whole point) of war.
You might think he means the destruction of the enemy and the enemy's cities and goods, but that is not what he means. Goldstein is saying that war is necessary to destroy wealth in the society that is waging the war. He says that this destruction makes it easier for the Party to rule.
The basic idea here is that the war destroys excess goods so that people are always really poor. If there were no war, they would have all sorts of luxuries and become comfortable. This would lead to them thinking about (and having the time and energy to) rebelling against the Party.
So, according to Goldstein, war is about destroying a society's own wealth so as to keep its population in line with the Party's desires. Here's a quote that shows this:
War is a way of shattering to pieces, or pouring into the stratosphere, or sinking in the depths of the sea, materials which might otherwise be used to make the masses too comfortable, and hence, in the long run, too intelligent.