In the context of online advertising, fill rate refers to the percentage of ad impressions that are successfully delivered to users. Fill rate is typically calculated by dividing the number of ad impressions that are delivered by the total number of ad impressions that are requested, and expressing the result as a percentage. A high fill rate is generally seen as a positive sign, as it indicates that an advertising campaign is delivering ads consistently and effectively to its target audience. A low fill rate, on the other hand, may indicate issues with the campaign, such as inadequate inventory or poor targeting, and can result in missed opportunities and lower revenue. Fill rate is an important metric for advertisers and publishers, as it can affect the effectiveness and profitability of an advertising campaign.