It's what kicks into action when something new is added to your hard drive, a file or a folder or a drive for instance, or when a new process starts up. It's the on-access scanner portion of your antivirus.
Unless you disable it, it will scan anything your download.
In addition to Colleague Ex_Brit excellent definitive answer. You can access all of your Products Features, and obtain most informative descriptions on how to use and to better understand all aspects of your protection.
Simply Open your McAfee User Interface, click on (Help), then click on ( Help) again. A Pane should open with your particular Suite/Others with a "Step by Step" guide. Or you can simply use this link: McAfee Help
Also as you surf, pay close attention to the Annotations your McAfee Site Advisor displays. As this is another facet of your Protection which informs you of Safe/Suspicious/or Dangerous Sites.
You also have at your disposal Superb (Free) Tools which can be obtained here:
All the very Best
McAfee Community Moderator