Dense is a liquid that tries to move down. Materials with Dense added will try to move particles above it, causing it to fall and giving the illusion of heaviness. Dense is not actually more dense than other materials.
Dense can be used to simulate convection. Dense + Gas creates a material that chaotically tries to crawl underneath of Water, even though Water is heavier and tries to fall back into position. Wall+Dense creates a pipe that sucks water up, or can be added to things that are supposed to sink. Changing downCoefficient in the Parameters can make a material that is gently or aggressively heavy. Dense can be used to create separate layers in fluid mixes. Light + Dense can be mixed to create a special material.