Clickers are a commonly used device on Olympic recurve bows and even used by traditional archers. Many barebow archers also use them for training.
What is it?
A clicker is an audible draw length indicator. Most work by sitting on the arrow and once you reach a certain length the clicker slides off the arrow and makes a CLICK noise. This is your indicator to release.
Why is this good?
When used correctly this gives you a consistent release point which helps reduce errors in impact position on the target. It's real main benefit though is it gives you a surprise release which can help reduce target panic and gold shyness.
Do they work?
Very much so, but they do take time to get the hang of. Every Olympian uses one, so they are considered essential for good results.