CofC Web Accessibility Guidelines- Rule (M)

(m) When a web page requires that an applet, plug-in or other application be present on the client system to interpret page content, the page must provide a link to a plug-in or applet that complies with §1194.21(a) through (l).

Translation: Plug-ins are programs you download to provide some kind of advanced function. Macromedia Flash and Apple Quicktime are common examples. If your page requires a plug-in you must provide a link to the get the plug-in. The link should be clearly labeled and easy to find.