Solids that are not decomposed remain in the septic tank. If they are not removed by periodic pumping, solids continue to accumulate until they over-flow into the drain field. This eventually leads to drain field plugging and drainfield failure.
The first signs may be slow draining fixtures, however the system may fail by discharging sewage effluent to the ground or back-up into the house as well (see below.)
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.
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.