If you’d like your ads to show on certain sites across the Internet, you can add these websites as:
Placements
Audiences
Keywords
Topics
Explanation:
Locations on the Display Network where your ad can appear. Examples include relevant websites and apps that partner with Google to show ads. A Display Network placement can refer to several things, such as an entire website, a subset of a website (such as specific pages from that site), an individual ad unit positioned on a single page, a video, a mobile phone app and more. You can choose specific locations by adding managed placements. You can also let Google choose relevant automatic placements for you based on your keywords.
You can choose specific locations by adding managed placements. You can also let Google choose relevant automatic placements for you based on your keywords or other targeting methods.
Read more here: https://support.google.com/adwords/answer/53215
