The diopter is the mechanism that allows you to adjust the focus for each eye individually, improving your viewing experience. You can adjust the diopter on a binocular by using the small wheel or lever that's usually located on the right side of the binoculars.
How to adjust your binocular's diopter
To adjust your diopter, first, focus your binoculars on something stationary and fairly close, like an obvious branch or telephone pole.
Close your right eye and adjust the main focus wheel to the sharpest view in your left eye.
Then, close your left eye and adjust the diopter wheel (not the focus wheel) to the sharpest view in your right eye.
Finally, open both eyes to check your adjustments. When your binoculars are perfectly focusing for both eyes, the object you're looking it should have a "3D pop" to its look.
Note: Generally, high-end binoculars will have a lock to keep your diopter in place, so you may need to slide your diopter wheel up to unlock and adjust it.
It's worth checking your diopter adjustment every once in a while to make sure it's exactly where you want it for the best views.