Thought to be as old as man, dowsing is used variedly to find water or gold, or to divine answers to questions. It's not clear how it works but theory goes that the person uses dowsing tools such as the pendulum as an extension of self, a fine-tuned antennae to seek out what one wishes to find, be it an item or an answer. People are divided on dowsing but dowsing has had some very famous fans. Albert Einstein was one such advocate.
The best way to test it is to try it. Ask a friend to bury something in the ground and walk slowly observing a response and changes in the tool. You may become a skeptic or a fan!