Targeting by topic is a good strategy if your client wants to:
- actively manage his budget because he has strict cost-peracquisition goals
- drive sales on his website
- control where his ads appear on the Display Network
- reach a specific audience
Topic targeting lets you place your AdWords ads on website pages about those topics, whether it’s agriculture, music, or something else entirely.Topic targeting allows your ads to be eligible to appear on any pages on the Display Network that have content related to your selected topics. As content across the web changes over time, the pages on which your ads appear can change with it. To display your ads on those pages, simply select one or more topics that you find relevant for your ads.
Just like your business caters to certain customers, your customers may be interested in certain topics. Topic targeting lets you place your AdWords ads on website pages about those topics, whether it’s agriculture, music, or something else entirely.
Read more here: https://support.google.com/partners/answer/2769377?hl=en