A Marketing Qualified Lead (MQL) refers to a lead where the potential customer has shown interest in a product or service of your business.  With the marketing efforts, they are more likely to become a paying customer as compared to other leads. 

They’ve engaged by performing actions like engaging with content, signing up for a newsletter, downloading a resource, or filling out a form. This is an indication that the visitor is ready to know more about your business and is likely to convert.  This indicated more targeted sales outreach.