Ad networks are platforms that connect advertisers with publishers. Advertisers want to promote their products, while publishers (like you) provide the space—your website, videos, or app—for those ads.
The ad network acts as a middleman: it delivers ads to your content and pays you a share of the revenue when people see or click those ads.
The most popular ad network is Google AdSense, but there are also others like Media.net, Ezoic, PropellerAds, and AdThrive.