that is where the pressure on the pedal is applied at the centre , it generally states that then your cleat and foot position is ideal on the pedal. in my case I am always + 8 to + 11 meaning my cleats are pressing on the outer edge of the pedal hence the power is not being distributed symmetrically .
this is the extract from Garmin on the PCO
Platform Center Offset (PCO)
The PCO measurement is calculated by identifying how force is distributed across the pedal platform during the pedal stroke. That means you can view and evaluate where force is applied relative to the center of the pedal platform and what the PCO distribution is over a given period of time. Analysing this data can help you determine proper bike fit and cleat position. It may also be helpful in preventing injury and rehabilitation.
PCO is measured in millimeters. Positive values (e.g., +6 mm) indicate increased force toward the outside of the pedal, while negative values (e.g., -4 mm) indicate increased force toward the inside of the pedal. You can view this information in graph form on your Edge device. The red line indicates the current 10-second average value and the blue line represents the average for the previous 30 seconds.