A dowser is another name for a water-diviner someone who can tell where there is water lying underground. The dowser uses a whole variety of tools to find the water, but one of the most common is the Y-shaped twig from a hazel tree. The dowser holds the twig by the two prongs of the Y, with the long stem pointing upwards. When the twig is above a spot where there is water beneath the surface of the earth, the stem of the twig will suddenly dip downwards, or in some cases it will twist itself violently out of the hands of the dowser.
Nobody knows exactly how this is done, not even the dowsers themselves, but it may be that they have some special powers which the rest of us don't possess - it is certainly true that only a very few people can do this.