This example shows the effect of Soft Source Radius on a light's reflection. Soft Source Radius is available in the Light category of the Details panel.
Soft Source Radius is a way to blur out a highlight that you feel is too sharp. Generally, it is a good idea to use the Roughness value or Texture of the Material itself rather than setting Soft Source Radius on a light, but it can be useful on the occasional stray light that has too sharp of a highlight. Also keep in mind that setting this to a value greater than one will make the light not physically correct.