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
Explanation: 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