A septic tank is a water tight concrete or plastic container that treats and stores domestic waste. The waste that enters the tank through the inlet tee, stores in the tank. The effluent water gravity feeds out the outlet tee, which is 2" lower than the inlet tee, to the drainfield. The drainfield acts as a cleaning process for the effluent water, eventually absorbing into the soil.
The level of the waste water should always be 2" underneath the inlet pipe. This is called "standing rate". Anything over the inlet pipe, "overfull", indicates the water is not draining properly through the outlet tee to the drainfield lines. This could be due to saturation of the lines, clogged filter or simply just a failing system. If the system is saturated from a leak or rain, pumping the tank can allow the drainfield lines rest for a few days. Pumping the tank will not remedy a failing system, it will only allow storage space for water usage until the drainfield lines can be replaced.