The TECS implementation provides a Total Energy Control System for fixed wing aircraft. Its outputs are throttle and pitch to achieve the desired sink / rise rates.
The default gains of TECS seldomly need to be tuned. There are two main tuning factors that allow easy adjustments:
FW_T_TIME_CONST- this is the main time constant. Reduce it to make the controller more aggressive (but also more susceptible to noise and less efficient).
FW_T_THRO_CONST- this is the throttle time constant. Reduce it to make the system correct altitude errors faster (but also be less efficient)