When a user opens a browser, which includes available inventory, his browser is sending all the details about the user in the milliseconds before the Ads are served, to recognize which Ad is the best to be displayed.


The browser is sending all the details about the user who is about to view the upcoming Ad. From user IP to geographic location, browsers, device information, and many more, this is the data that is used for targeting. 


This way you can ensure the right people will be viewing your Ad.