Many people will have no idea what Silverlight actually is or what it does but for the rest of us it was the Microsoft’s attempt to compete with Adobe Flash which was never really unseated as the premier platform for Internet applications. Silverlight did see some usage in Windows Phone 7 but its future has been somewhat murky as a result of Windows 8 appearing on the horizon for both tablets and smartphones.
However, the lowdown is that the writing has been on the wall for a long time. Developers have known that the Windows Presentation Foundation which is one of the building blocks for WinRT (Windows Runtime) for Metro-style apps under Windows 8. Making the transition should be relatively painless but may require some work to port over applications.
For everyone else who is not a developers, the main thing to understand is that you can probably ignore Windows Update bugging you to install Silverlight under the optional updates section all the time. Most of us will not have a need for it and perhaps never will at all.