A Clicker or some type of "marker" is used as a Conditioned Reinforcer. Its a signal that tells the dog that they did something right at that moment. This really comes into play when you are doing off-leash training and are not close enough to reward immediately. For example, If you have your dog in a sit position and you are about 30 feet away and you give the command to "down". The dog goes down and you walk in to reward. In the meantime, the dogs sniffs the ground, looks up at a flying bird, licks their butt and then you are close enough to give the reward. What actions does the dog interpet for getting the reward? Most lickely, it was the butt licking. Even if you are only taking a treat from your pocket and reward, the dog could do something that misses the behavior you want to reward.
So the clicker or "Marker" will tell the dog, they did it correctly and a reward is on its way. Look at it as if you are taking a snapshot at that moment of the exact behavior you want. I don't use a clicker, but my "Marker" is the word "Yes".
Getting the dog to associate the Marker with a reward for doing something correctly, you need to start small. You might want to use the "Name Game" as a way of getting "Marker" into use. With the dog, very close to you, wait until they are looking away from you. Then call their name. Once they look at you, Mark it with the clicker or word and immediately reward them. Reward everytime you do this at first for them to associate the marker with the correct behavior and the reward. After a while doing this, the association is built. Then move to some more easier commands to work with the "Marker".
Eventually, you will be able to "Mark" the wanted behavior and the dog will know this and then you can reward them and the dog will know what the reward is for when you can't reward immediately.
I hope this helps out or atleast explain the concept.
