What is Silverlight Runtime?
Silverlight CLR is a subset of the .NET Framework that contains components and libraries, including data integration, extensible Windows controls, networking, base class libraries, garbage collection, and the common language runtime (CLR).
Silverlight Runtime is a browser plug-in to support Silverlight enabled applications. If Silverlight runtime is not installed, browsers will not be able to run Silverlight elements in the browser. You can set up your Silverlight tags such a way that your browser will automatically prompt the user to download and install the Silverlight plug in when your application is launched in the browser.
Installing the run time is a onetime operation on the client. Once installed, it will be automatically launched when any Silverlight application is loaded in the browser.