In binoculars, IPD is the distance between the centers of the exit pupils. It is measured in millimeters. Typically, a pair of binoculars shall have the same IPD as your eyes so that you can be comfortable while glassing. As such, binoculars need to have an adjustable IPD.
The ATN BINOX 4K 4-16X Smart Ultra HD Day/Night Vision Binoculars is one of the best binoculars with excellent interpupillary adjustment. Read more about this pair here.