In a gravity drain field system the clean wastewater (hopefully not grease and solids) leaves the tank via overflow pipe that is usually 16" or so below the top the scum/grease layer. http://inspectapedia.com/septic/tank.gif
When the scum layer or sludge are to large solids can leave the tank and flow through to the drainfield. The drainfield is composed of perforated pipe and gravel (round rock or crushed).
The gravel creates a porous evironment for microbes to grow. These microbes filter and digest some of the nutrients in the wastewater. Microbes also grow at the soil gravel interface. When scum and solids get into the drainfield they plug the perforated pipe (occasionally) but mostly the gravel pore spaces. This prevents wastewater from seeping into the soil below the drainfield. It then backs up in the drainfield lines and comes to ground surface in complete failure cases. There is really no way to unplug the gravel pore spaces.
The solution is pump the tank before scum and sludge layer each exceed 1/3 of tank. Also install a biofilter on tank exit pipe.