Display advertising refers to the ads that you see when browsing websites usually when reading articles or funnies or general blogs. There are over 2 million websites on the Google Display Network so there's a good chance that you've even seen a Google Display Network ad today.
Unlike search ads where ads are served upon someone typing in a question, display ads do not appear on the search results page and appear as a banner or banner image when you are on a website. For example if you visit https://www.realestate.com.au/ you will see a couple of display ads on the homepage. At the time of typing this FAQ, a couple of the ads displayed are Google Ads which means this website is on the Google Display Network.