A request to display an ad (ie when a site requests ads for display). It's part of the selection and serving process
A user's web browser or mobile device loads an ad tag (on a site) or app code (app) that create/triggers an ad request send to the ad server to select and serve and ad
An ad request includes:
Example of data send with a request:
It's the number of ad units that requested ads (for content ads).
More see the dedicated page: Google Ad Manager - Ad Request
for bidding