When you access a site such as youtube your computer needs certain things to run the application on that website. You may have things such as flash, that need to be installed before you can view content such as videos or animation features on sites. These help you access and download content for better viewing of the site.
examples of some plug ins:
RealPlayer: enables your computer to play streaming RealVideo and RealAudio.
Adobe Reader: For viewing and printing Adobe Portable Document Format (PDF)
Adobe Flash Player: is the universal rich client for delivering effective Adobe Flash experiences across desktops and devices.