Soft Selection lets you sculpt geometry in an organic way. It’s useful for sculpting a smooth object or making smoothly integrated slopes or contours to your model without having to transform each vertex manually.
Soft Selection works by maintaining a falloff from the selected components to the components around your selection to create smooth transitions.
For example, if you select a vertex at the center of a plane and translate it up into a spike without Soft Selection, the sides are very steep and the point is very sharp. When Soft Selection is on, the slope and tip are smoother.
You can use Soft Selection with polygon meshes, NURBS surfaces or NURBS curves.