Depth of field is the area that is in focus between two point when you look through the eyepiece. For example , when you focus precisely on an object at infinity, you may find that other targets are also in focus down to 50m. So in this example the depth of field would be 50m – Infinity.
As a rule of thumb, binoculars and telescopes with lower magnifications have the largest depth of field. 7×50 binoculars are the best example. With these you can set on around 30-50m and use almost focus free. On the flip side high powered binoculars have such a small depth of field, they need almost constant adjustments of the focus wheel to keep everything sharp.