Galvanic corrosion is generated the flow of electrical current between dissimilar metals immersed in an electrolyte such as seawater.  As ions of the negatively charged metal flow out into the water, the metal itself is consumed.  The best protection from galvanic corrosion is installing anodes of a more highly charged material such as zinc, aluminum or magnesium.  The type required for each vessel varies (check your manual or dealer), but three types are commonly refered to as Zincs.

In this video Eddie takes us through replacing Zincs on a Mercury Bravo Three.