Your PD is the distance from the center of one pupil to the center of the other in millimeters. This information is necessary to properly center your prescription correctly in your frames. It's important to note that the average PD is between 57 and 65mm. You may have two PD values - one for each eye. It is best to get this measured by your eye care professional. If your doctor did not include your PD measurement with your prescription here are a couple of options:
Use the free app called Eyemeasure. (This option does require an iPhone X or newer - not Android compatible)
You can also measure it yourself by following these instructions: How to measure your PD