The Unsharp Mask effect increases the contrast between colors that define an edge.

Original (left), and with effect applied (right)

Radius

The distance from the edge at which pixels are adjusted for contrast. If you specify a low value, only pixels near the edge are adjusted.

Threshold

The greatest difference between adjacent pixels for which contrast isn’t adjusted. A lower value produces a greater result. A value that is too low causes contrast for the entire image to be adjusted and can generate noise or cause unexpected results.