What is it?
Gross Simplification: Silverlight is like Adobe Flash. Some web
pages use Adobe Flash to show animations, video, menus, etc. Some
web pages use Silverlight to show animations, video, menus, etc.
Can you uninstall it?
Yes. You can uninstall almost anything you want on your machine. You
could start running Ubuntu instead of Windows. You could hack
(or obtain a hacked version) OS X and probably get it to run on
your system. So in the end anything you don't want - as long as
you are willing to lose whatever would be allowed due to the
presence said thing - you can remove. In this case, perhaps this
will help:
http://www.microsoft.com/getsilverlight/get-started/install/uninstall-win.aspx
By the way - you can find information like this via Google (or
other) Internet searches.
http://www.google.com/search?q=define:Silverlight
and
http://www.google.com/search?q=how+do+I+uninstall+Silverlight
with a little reading through (scanning) of the search results can
lead you to essentially what I told you in a few minutes. ;-)