The Sharpen effect layer adds contrast to the visible edges on your canvas. Add a sharpen layer by clicking the Sharpen icon on the Effects tab of the toolbar, then adjust the selected sharpen layer in the Sharpen Properties panel.
Usage
A photograph which is subtly out of focus can sometimes be rescued by sharpening visible edges through an increase of contrast.

Sharpen Properties
Use the Amount: slider to increase the sharpness around visible edges.
Use Advanced Settings
This disables the slider control and enables the other controls in the panel. This also changes the method being used to perform the sharpening to an Unsharp Mask. This allows you to fine tune how the sharpening is applied to the image, and can give a more natural look.
Radius
Smaller values will sharpen the finer details of an image. As the size of the image or the distance you view the photo from increases, you will want a larger radius.
Amount
How much sharpening is applied. This can be very image dependent, so it can take some adjustment to find the level that you think is best for each image.
Threshold
Controls how distinct an edge must be before sharpening is applied to it. This can help prevent sharpening of things like noise and grain within an image.