Microsoft introduced Silverlight as a technology for media rich applications to run inside a browser but also outside. However it has not been announced that it will not be supported beyond 2021 so big users like Netflix will need to migrate, hopefully to HTML 5 based technologies.

So, for now, if you need to install Silverlight or want to confirm you have the latest then head over to Get Silverlight | Microsoft Silverlight. If a version number is displayed, then you have the latest, if you get an "Install" button then you don't and you need to update.