The Unsharp Mask effect increases the contrast between colors that define an edge.
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.
From the Adobe Help Page (https://helpx.adobe.com/after-effects/using/blur-sharpen-effects.html#unsharp_mask_effect)
Image from http://www.unsplash.com