Interpupillary distance (IPD) is the distance between the center of the pupils of the two eyes. IPD is critical for the design of binocular viewing systems, where both eye pupils need to be positioned within the exit pupils of the viewing system.
This is important to know for setting your headset with the proper IPD to ensure your display is focused and to minimize your eyestrain.
You can get it measured by your eye doctor, or using a mirror and a ruler, but I came across this demo which lets you find your IPD using a webcam and a standard sized bank/credit card.
WARNING: if you’re going to try this, be aware that you’re granting access to a website to access your webcam and allowing it to take a picture of your bank/credit card so beware.
An even better method is to walk up to the chaperone grid fence. Adjust the headset up and down on your face until the horizontal lines are sharpest. Then adjust IPD until vertical lines are sharpest.