Question

What is the depth of field​

Answer 1

Answer: Depth of field is the area in a photograph that appears in focus. Specifically, it is the distance between the nearest and farthest objects in the photo that appear acceptably sharp. A photograph can have a shallow depth of field, meaning that only a small portion of the image is in focus, or a deep depth of field, meaning that most or all of the image is in focus. Depth of field is affected by factors such as aperture, focal length, and distance from the subject.

Answer 2

This is what the depth of a field is:

Depth of field is the portion of that distance or 'depth' that is 'in-focus'. A higher depth of field would see the whole image from foreground to background sharp and in focus, a lower depth would result in blurry backgrounds and blurred elements in the foreground too.

(Credits to FXhome.com)

Or, in math the depth of a feild is this (screenshot below & credits to Wikepedia)

Hope I could help