In the context of display advertising, a bid request is a request made by an ad exchange to potential advertisers to submit bids for the opportunity to display an ad to a specific user on a specific website or app. The ad exchange typically uses information about the user, the website or app, and the available ad space to determine the value of the ad impression and sends the bid request to advertisers who may be interested in reaching that user. The advertisers then evaluate the opportunity and submit a bid based on how much they are willing to pay for the ad impression. The winning bid is then used to determine which ad will be displayed to the user.

When an ad exchange sends a bid request to potential advertisers, it typically includes information about the user, the website or app, and the available ad space. This information can include the user's demographics, location, browsing history, and other factors that may be relevant to the value of the ad impression. It can also include information about the website or app, such as its content, audience, and traffic. Additionally, it can include information about the ad space, such as its size, location, and format. This information is used by the advertisers to evaluate the opportunity and determine how much they are willing to bid for the ad impression.