The Curves dialog shows a graph with a diagonal line going from the bottom left to the top right. This graph is a depiction of all the tonal values in your image going from pure black in the lower left corner, to pure white in the top right corner. All the gray tones in-between are represented by the sloped line.
To bring our image from blurry back to sharp, we want to increase the slope of this diagonal line so that the degree of change between pure white and pure black is reduced. However, we don't want to make the angle perfectly vertical, or else we'll put the image back to the jagged appearance that we started with.
