The drainfield is where liquid from the septic tank flows through pipes in your yard for final treatment by organisms in the soil. Grass and/or shallow-rooted plants are the best covers for your drainfield. See Landscaping tips with a septic system for more informaton.
Don't park cars and trucks on the drainfield or septic tank. Also, don't install driveways, patios, carports, decks, storage sheds, sports courts, landscaping plastic, and/or allow animals to graze. These activities pack soil and may lead to pipes breaking.
Your yard should also have a "reserve area" which is where your replacement drainfield will be located if your drainfield fails.
See brochure on "Understanding and Caring for Your Septic Tank System" by the Washington State Dept. of Health.