I can't tell much from your question, but I'll present two main ideas and you can feel free to narrow your inquiry from here. There are two sides to passing parameters with IFrame module - 1. to Iframe module or 2. to IFrame TAG. The main difference is that while the first one passes parameters to the module to be used in CONSTRUCTION of the IFRAME tag, while the second one is passing parameters TO the Iframed's PAGE to be used by that page's code or script.
Imagine you have many documents - for example, manuals for some products you are selling. And you want to display the manual following user's clicking a link to that document. You might not want to link directly to that document because you don't want to rely on browser plug in to display it, and you don't want browser to take over the navigation (once inside the plug in, there are no menus for your website, for example). In this case you may want to set up an IFrame module on some page, and have it read a querystring parameter that contains the name of the document. This way you would be able to display many different documents using the same page, instead of having many many pages that do the same thing.
Example: http://www.yadavraham.org/default.aspx?tabid=29 - the two right columns link to a page that is set up that way. now, I didn't use core IFrame on that site - it was long ago and core module didn't support this. but it still illustrates the point.
You want to display some page inside the iframe that has a certain functionality that depends on - may be you want to tell that page a bit about the user (by passing username, for example, or his/her timezone...) or, may be you want to pass some other information - in either case, these parameters can only be passed as part of a SRC attribute of the IFRAME tag and will be in the plain view of anyone who is willing to click on View Source in a browser's menu. So, you have to be careful about what kind of info you are passing.