Wordpress: How does a plugin generate a page?

Question

Could someone please solve a beginner's crisis for me? I simply CANNOT figure out how a plugin generates a page or even multiple pages without making a page template in the dashboard. I have read Google results and even talked to the author of this plugin I am reading through to get an idea but the way he explained it was too complex for me to understand. Here is a link in case you want to see his explanation. But I just want to understand...If a plugin is ONE PHP script, then whatever function calls the HTML template for the page must include() it right? At least that's how this plugin does it. But then why is it that when I click on one of the pages for the plugin I see this in the URL: http://www.efficientmind.org/?page_id=73. What does the ?page_id=73 mean? And how does query_vars play a part in all this? I'm so confused...Could someone maybe explain it in layman's terms and then also in a more technical so I can compare and contrast??