Depth of field can be explained as the amount of your scene
that appears sharp and in focus.
This area extends in front and behind the focus point but its not
distributed evenly, extends twice as much behind as in front of it.
The shorter the focal length the greater the depth of field and the
longer the lens the small it will be.
The aperture will also affect your depth of field a small aperture of
f/16, f/22 will keep as much of the scene as sharp as possible where
as a an aperture of f/4, f/2.8 will mean that much less of the scene
will be in focus.
Distance, the closer you are to the subject the less your depth of field
will be.
For landscape photography in order to obtain as much of the scene in
focus as possible, select a small aperture about f/16 on full frame
cameras and f/11 on aps-c cameras, focus on an object about 1/3 of
the way into the scene.