Interpupillary distance (IPD) is the distance between the center of the pupils in your eyes.
A quick way to get an estimate is by facing a mirror and holding a millimeter ruler against your brow. Use this measurement as a guide to adjust the distance between the lenses of the headset so that you'll have a better viewing experience.
To change the distance between the lenses, rotate the IPD knob.
As you adjust the IPD, you'll see information displayed in the headset
that indicates the current distance between the centers of the lenses.