This is going to be very quick post. Yeah..I didn't post a lot.. Thanks for noticing.
Don't ask me why it's called an unsharp mask eventhough it's function is to sharpen pictures. Please ask uncle Google for a detailed explanation of the process.
I don't really sharpen my pictures a lot. In contrast, I'll purposely decrease the sharpness of images by around 15% in either Photoshop or Lightroom. Maybe it's because I like my photos a bit soft..
Anyway, just to demonstrate the effect of the mentioned filter, here are the original and post-filter-application photo.
Unsharp mask applied
As you can see from the above example, the filter does sharpen the image. What settings am I using? The magic numbers are Amount: 130, Radius: 1.5, Threshold 6.
Have fun unsharping your images.