Refers to that clip in a movie/tv show/commercial where a something happens and we see a dog react to it by turning its head to the side to look at it, tilting its head to one side as if confused, or its ears pop up as if to say "wow".
Can sometimes be accompanied by an "arooo?" sound.
Other *animal* reactions exist, most frequently cats and monkeys, but are much less common.
Jason and Geoff are watching a movie.
Jason: "Holy crap, sweet dog reaction!"
Geoff: "Haha, yeah, sweet. That dog is the best dog actor."