It is how your course is displayed on the dashboard page for all users. See below for an example and the setting that controls it's appearance,
Within Moodle, navigate to your course and click the settings gear for the course itself and click 'Edit settings'
Using the Card example above, the 'Course Summary' section is what is displayed for #2, 'Course Summary'. Any text that you type into this field will be displayed underneath the picture.
Using the Card example above, the 'Course Summary Files' section is what is displayed for #1, 'Course Summary Files'. Any image that you place here will be used as the card image.
Scroll down once you have edited the sections you wanted to change and click 'Save and Display'. Now navigate back to your 'Dashboard' page and refresh the page, your new description and image for the course should now be displayed.