Molly might conclude that anybody who fills out her online demonstration request form is an MQL.  Another company might set the bar to MQL qualification at something involving a combination of viewing specific pages, interacting with certain forms, and opening a certain number of email messages. For that kind of analysis, we recommend marketing automation software.
If you're wondering what a sales funnel is, simply imagine a real-world funnel. At the top of that funnel, some substance is poured in, which filters down towards one finite destination. In sales, something similar occurs. At the top, lots of visitors arrive who may enter your funnel. However, unlike the real-world funnel, not all who enter the sales funnel will reemerge out from the other end.