The number of times that the code for creatives has been served to users during the specified date range.
The code serves metric is only relevant for interstitial or video creatives. For these creative types, there might be more code serves than impressions, because the creative isn't displayed when the webpage is first loaded. Instead, Campaign Manager first serves the code, which tells the browser when and how to display the asset (the piece that users actually see). A code serve is counted when the webpage loads, but not an impression. The impression is counted only when the asset is requested by the browser. In some cases, the code tells the browser to wait several seconds before the asset is shown, so it's possible that users will close their browsers or navigate to other webpages after the code is served but before an impression is counted.