What is use Siverlight and the use of it?

Silverlight is a development platform for creating applications that run in a Web Browser. These applications can be for the Web, Desktop or/and mobile devices. Here [^] is the Microsoft Silverlight Web Page.

Hope this helps

Shortly Silverlight is subset of WPF(Windows presentation foundation which is the Presentation framework for desktop apps)
Silverlight runs in browser and used to create rich GUI for browser based apps.
You can search on internet abt this tons of helping material is there.

Just pick up a book on Silverlight and start reading.
That should help you understand the concept behind it.