When on, Use Soft Select affects keys adjacent to a selection set of keys according to a falloff threshold. This applies to both Curve Editor keys and Dope Sheet Edit Keys modes.
To adjust the Soft Selection range and falloff, choose Soft Select Settings on the Keys menu.
To soft-select keys in Track View:
The Soft Selection dialog opens; by default, it's a toolbar at the bottom of the Track View window.
Alternatively, you can toggle soft selection with the Soft button on the Soft Selection dialog/toolbar, and you can make the selection after you turn on the feature.