This TechNote has the details:
Basically, you define the variables and their values within an optional attribute of the HTML <Object> tag called FlashVars. The <Object> tag is the one you use to embed a SWF file in a web page. Then you use ActionScript to access the variables within the SWF file.
Peter DeHaan also has a nice explanation of how to do this here: http://blogs.adobe.com/pdehaan/2006/07/using_flashvars_with_actionscr.html