What is the trimmed condition of an airplane?

The trimmed condition of an aircraft means that with constant power and no control inputs, the aircraft will fly straight and level (no climbing or descending).

Trimming an aircraft means to configure the aircraft to equalize positive and negative forces on the lifting surfaces. An elevator trim control (a small wheel or switch) allows the pilot to move a small tab on the trailing edge of the elevator. The position of this tab (up or down), will alter the airflow just enough to balance the lift and weight forces and keep the airplane flying level without any input from the pilot. Some aircraft also have rudder trim to counter yaw created by engine torque at different power settings.

If an airplane is not trimmed correctly, it is not being flown efficiently, and the pilot is either constantly pulling back on or pushing in on the control stick/yoke/wheel to keep the aircraft at a steady altitude.