The “depth of field” of a pair of binoculars refers to its ability to keep objects further towards the background and closer towards the foreground in focus without you having to adjust the focus. Stated in another way: Once you’ve focused on an object in the distance, how far behind this object (in other words, further away into the background) and how far in front of the object (in other words, closer to you, the viewer) is still in focus, without you having to adjust the focus.
This is another feature of a binocular directly determined by magnification: The more powerful the instrument, the more restricted the depth of field.
Really powerful binoculars, like the ones used for astronomy, have very little depth of field and getting the focus right is very difficult. Fortunately the object (stars) does not move, but watching moving objects like animals and birds will be increasingly difficult with a pair of binoculars with high magnification. You struggle to get the focus right, then the bird starts flying towards you and the image is blurred. On the other hand, with lower magnification like 8x or 7x, the depth of field is deeper. For this reason binoculars recommended for birding are usually not more powerful than 8x.
Depth of field becomes more sensitive the closer you focus, but opens up the further you focus.
A special category of binoculars with super depth of field features is the so-called “auto focus” binoculars. Please consult my article on Auto focus binoculars for a discussion of this category of binoculars.