Refer to figure.
It's important to check that important flight instruments such as the direction indicator (DI) are indicating correctly before getting airborne.  The DI has to be manually synchronised with the direct reading magnetic compass by pushing in and rotating the caging knob on the instrument.  Once synchronised it's serviceability can be checked by observing the indications during turns while taxying.

The instrument display consists of plan view of a fixed aircraft symbol with a lubber line represent the nose of the aircraft. This fixed display is superimposed on a 360° rotating  compass rose.  When the aircraft turns to the left the compass rose rotates to the right and the heading indication decreases, when turning to the right the compass card rotates to the left and the heading indication increases.

during a turn to the left the headings should decrease  →  CORRECT   For example if the initial heading is 180° al left turn will cause the heading indication to decrease through 170, 160, 150 etc

during a turn to the left the headings should increase → INCORRECT.  In a left turn the heading indication decreases

during a turn to the right the headings should decrease → INCORRECT.  In a right turn heading indications increase.