The content network was the ad distribution network for Bing Ads running in Windows apps. It was available only in the United States.
Bing Ads stopped serving ads on the content network in 2017. If you had ad groups with content ads, they were automatically turned into search ad groups (for search & content ad groups) and paused search ad groups (for content ad groups).
If an ad for an clothing company had the keyword "dress" and the keyword "shoes" and there was a content network app related to fashion, the ad could display even though a person viewing the app hadn't searched for "dress" or "shoes".