Zero reset (zero offset) calibration is an important feature that resets the zero offset value for the power meter sensors. There are physical and environmental conditions that may affect the zero offset value and there are methods both manual and automatic that will adjust this value to accommodate for the changing physical and environmental condition.
The zero offset of the power meter is essentially the sensor reading or values measured when the power meter has no pedaling load (torque) applied. The act of calibrating the zero offset causes the power meter to measure the value at zero load and then records this value as the baseline for power measurement. Loads applied while pedaling will then be measured as torque and used by the sensor to determine power in Watts.
The zero offset value can be affected by the installation of the crank arm and the tightening of the securing hardware. The torque applied to the securing hardware can impart some strain into the crank material that is easily accounted for by manually calibrating the zero offset.
Any time the power meter is removed from the bike and reinstalled the zero offset should be calibrated.