Interpupillary distance (IPD) is the distance between the pupils of your eyes measured from center to center when your eyes are focused on infinity. IPD varies from person to person, and is also smaller when looking at something nearby. The same term also applies to the distance between the centers of a binocular’s exit pupils.
To take full advantage of the field of view and light from your binoculars, the binoculars' IPD should be the same as your eyes' IPD. Because IPD varies between people and uses, virtually all binoculars have adjustable IPDs. In general, lower-power binoculars with smaller objective sizes can be set at smaller IPDs.
If you don’t have the IPD correctly adjusted, you’ll only see part of the scene you’re looking at due to the beams of light from the eyepieces partially missing your pupils. Adjust IPD by moving the two tubes of the binoculars towards or away from each other. Some binoculars have a scale on the center pivot to help with proper IPD adjustment.