ACR stand for automatic content recognition. The process is used to recognize audio content on radio - the precursor was Shazam - or TV content. In both cases, the recognition is based on audio content trough audio fingerprinting.
In digital marketing context the first use was to give radio listeners the possibility to buy online music or merchandise tied to the broadcasted song.
ACR is also used - nowadays, it is the main usage - to tie in real time TV programs and commercials to online content and to develop interactivity.