An amount the advertiser is willing to pay for ad space
The amount of money an advertiser is willing to pay for advertising space within a platform (e.g. Facebook) or an app. This amount will then participate in an auction, so it does not necessarily determine how much the advertiser will actually spend. Usually, the highest bid gets the most ad placements, but that is not the only determining factor:
Your bid - The maximum amount you are willing to pay for a bid. Platforms and ad networks are designed to allow you to pay the lowest possible amount. For example, if your highest-bidding competitor bids $1 and your bid $2, you will only pay $1.01 to win the bidding auction.
Relevance - Your ad's relevance score is important for certain platforms, such as Facebook. The score is determined by positive and negative interactions users have with your ad, such as comments, clicks, hiding, and reporting the ad.
Estimated action rates - if the advertising platform estimates your ad to receive a high number of impressions, it is more likely to give your ad priority over competing ads.