A marker can be any sound that has been paired with a primary reinforcer such as food so that it can precisely indicate to the dog that what it has done at that point in time will earn reinforcement. A marker can be a word or a sound such as a whistle or a clicker.
What anyone who has done training with food rewards knows is that it works. Dogs who get treats for doing things tend to do those behaviours more often in the future. However, just dishing out cookies for stuff that the dog does is imprecise. Let’s take a sit for example. The dog sits and you give it a treat. It sat, right? But what exactly did your dog associate with getting the treat? She knows she got a treat and can guess it was for something she did in the last several seconds. But there can be a lot of choices in this time frame. It could be that she was looking at you before she sat; it could be movement of her feet; what about standing up immediately her rump hit the floor? Given enough repetitions she would probably work out which of these options is present most often. And in the end, she would probably figure out that sitting was what you wanted her to do.
But the question now is what did the sit look like? Was it a nice square sit? Or did she sit on her hip – a sloppy, puppy sit? She might start out with a square sit but quickly collapsed into a sloppy sit which just happened to be the time when she got the treat!
On the other hand, if you use a well-timed marker, she will know that it’s about her rump hitting the floor because each time she does this the marker always happens just then and the marker predicts a reward is coming. This is much clearer communication with your dog. Consistency and Clarity.