On the Google Display Network, your ad is eligible to show on a webpage if your:
- keywords match that webpage’s content
- ad text matches that webpage’s content
- landing page matches that webpage’s content
- website matches that webpage’s content
AdWords has two main networks: Search and Display. The Adwords Search Network reaches people when they’re already searching for specific goods or services. The Display Network helps you capture someone’s attention earlier in the buying cycle. For example, if you run an art supply store, you can catch a mom’s eye when she’s reading reviews about the best brands of washable paints, but before she puts her toddler in the car seat and heads out to buy.
Google’s system analyzes the content of each webpage to determine its central theme, which is then matched to your ad using your keywords and topic selections, your language and location targeting, a visitor’s recent browsing history, and other factors.
Read more here: https://support.google.com/adwords/answer/1726458?hl=en