A bidder is a person or entity that submits a bid in response to a bid request in the context of an auction or bidding process. In the context of display advertising, a bidder is an advertiser who submits a bid in response to a bid request from an ad exchange for the opportunity to display an ad to a specific user on a specific website or app. The bidder uses the information provided in the bid request to evaluate the value of the ad impression and determine how much they are willing to pay for it. If the bidder's bid is the highest, they win the auction and their ad will be displayed to the user.