SMIL is a specification defining a set of elements that control animations. This is called declarative animations.
FakeSmile is a SMIL implementation written in ECMAScript. What people call a polyfill. It is primarily targeted to SVG animations.
While some browsers already support animations (Opera, Firefox, Safari and Chrome), others don't. While I'm writing this, only MS Internet Explorer is lacking this feature. But they all support programatic animations via scripting. FakeSmile makes declarative animations work in IE too.