Basically it deals with large images, compression, and panning/zooming/etc. A better explanation of what it is:
http://www.mass.gov/mgis/mrsid.htm
http://www.lizardtech.com/ was producing the plugins, but they have opted not to support it on the Mac.
A lot of places use this compression/setup for handling certain types of images (maps are a big part of that).
I can load OS 9 and use the Classic plugin under Internet Explorer, but it's slow, and a bit unwieldy.