This simply means that if you click on certain links can activate your web browser so that once the sound of video file downloads, it will automatically play the sounds for you. Not all Web Browsers have built in sound support so if you don't have Internet Explorer 3.01 or Netscape Navigator 3.01, then click here to download it. These latest versions have the sound capabilities built into them. If your computer is Audio Compatible, then you would have heard the sound. If not then you need to visit your web browsers web site and find out how to make it work.
Video gets a little trickier.
A Plug-In is a program which works in conjunction with your web browser to view video files. In you need. This pretty much covers the World Wide Web on a basic level.