Depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.
In cinematography, a large DOF is often called deep focus, and a small DOF is often called shallow focus.
Depth of field is affected by camera to subject distance, focal length/ lens used and Aperture.
Focal Length/ Lens
A wide lens i.e. 24mm (Zoom out) creates the greatest depth of field (more things are in focus) while a longer lens, i.e. 105mm, (Zoom in) creates a shallower depth-of-field (typically just the subject will be in focus).
The more open the aperture the more shallower the depth of field will be.
'The important thing to remember is this: the higher the f number, the more things in front of and behind the subject will be in focus, but the more light you will need. The lower the f number, the more things in front of and behind the subject will be out of focus, and the less light you will need.
Consider the effect of Depth of field and how it helps your storytelling...