Dispersants are chemicals that help break up fresh oil slicks into much smaller droplets. They are generally used to reduce the oil's potential impacts on shorelines, birds and other surface dwelling animals, and to hasten biodegradation of oil.
Crude oil is an organic substance and, as such, is largely biodegradable. Given enough time, ocean turbulence will reduce an oil slick from a large cohesive film to separate droplets, which naturally occurring bacteria break down into less toxic substances. Dispersants help speed up this process by reducing the oil molecules' ability to stay together in the large mass of a slick.
Oil slicks have a relatively small molecular surface area for bacteria to attack. Smaller, dispersed droplets have an overall larger surface area. Think about a large tree that is cut down. Left untouched, the tree will take many years to fully decompose. Run this same tree through a chipping machine and the resulting mulch will quickly break down into compost.
Dispersant formulations usually include a solvent (one substance able to dissolve another) and a surfactant (a wetting agent that lowers the surface tension of a liquid, allowing easier spreading). The solvent helps the surfactant penetrate the oil so it can break up the slick into droplets that move from the sea surface into the water column.
The benefits dispersants bring to surface waters can become detriments to the water column and is an issue that must be considered before they are used. Increasing the oil's surface area for bacterial attack inherently increases the surface area of the oil's toxic components that are exposed to the surrounding environment. Moving the droplets from the sea surface to the water column limits or eliminates the oil's contact with surface dwelling animals, but increases the exposure to undersea life. Dispersants also bring their own toxicity to the environment. Overall toxicity depends on the concentration of a substance and the length of exposure.
Health Fact Sheet from the Centers for Disease Control and Prevention