An associated image is an image linked to your page information, and will show in many places where your content is shared.
All page types include the option of adding an 'associated image'. This is the image the will accompany the link to your site from your dynamic overview page, and other places throughout the University website. it is part of the data for your page, which also includes your title and summary.
You can see associated images being reused on overview pages throughout this site: