What is a clicker?
Clicker is a small and very useful training tool. Usually, it consists of a plastic box roughly a size of a thumb and a clicking mechanism. There are two types of clickers: with a button and with a metal tongue. I prefer the button type, because they are quicker and more reliable.
Clicker is used to mark the desired behaviour as it occurs. You cannot always give the dog treats (unconditioned reinforcement) exactly at the right moment, but you can use a clicker (conditioned reinforcement) to indicate to the dog that you like this behaviour.