Instead of actually giving back a portion of the money the company invested for advertising as a penalty for not delivering, the network typically gives them ADUs, which are basically additional commercials that make up the difference.
An ADU here or there is no big deal, but the more each network has to give away, the more future revenue it loses.
An ADU is an Audience Deficiency Unit.
Basically, it means the advertiser bought a commercial, or more likely group of commercials, and was promised a number of viewers the network didn’t deliver.