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
see which images and text ads within your campaign are performing best on the specific websites you are targeting
run a report to determine which keywords are most likely to convert based on the past 30-days of your campaign
see potential webpages where your ad can appear based on your keywords
managed CPC bids for contextual campaigns within your account
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