In the real world, many materials are translucent to some extent, and do not immediately reflect light at their surface. Instead, light penetrates the surface, and is scattered inside the material, before it is either absorbed or transmitted. This effect — called subsurface scattering — can be used to create a variety of translucency effects and also to enhance the realism of a wide variety of rendered materials, even when used sparingly.
The Fast Subsurface Scattering shaders simulates the appearance of subsurface scattering, but not in a physically correct way. As such, they allow you to tune the scattering effect by eye, rather than requiring real-world scattering and absorption coefficients (not to mention photon lighting) like with physically accurate subsurface scattering (misss_physical_phen).