One of the ways ads are targeted to AdSense publisher websites is through placement targeting. Placement targeting allows Google advertisers to choose specific ad placements where they would like their ads to appear. An ad placement can be an entire website or a specific sub-set of ad units within that site, such as only ad units on a site's sports pages or all ad units at the top of the page.
Advertisers find ad placements in several ways, including by listing websites where they would like to advertise or by searching for placements that match the themes and topics they would like to target.
If your site is part of the AdSense network, it should automatically be visible to advertisers as an available ad placement when they search for themes or topics related to the content of your site. You can also define your own ad placements using specific sub-sets of ad units on your site.
Learn more about defining your own ad placements.