You can use Display Planner to:
- see how other advertisers perform on websites where you want your ad to appear
- compare how your current Display Network campaign could perform on websites you’d like to target
- see ways to reach your target audience based on your keywords, website, or interest categories
- see which image and text ads are performing best on the specific websites you’re targeting
Display Planner generates ideas for all the ways you can target the Display Network. Targeting ideas are based on your customers’ interests or your landing page. They include keywords, placements (websites, videos, mobile apps, mobile app categories), topics, interests (affinities, in-market segments), demographics (age, gender), and remarketing.
Display Planner shows you the potential reach and cost of targeting ideas to help you plan your Display Network campaigns.You enter keywords, topics, or websites that you think are likely to match your customer interests, and your landing page. That will get Display Planner started with the types of targeting and keywords that can build the foundation of your campaign.
Read more here: https://support.google.com/adwords/answer/3056115?hl=en