It is the optical property of lens;

aperture in a lens is adjustable, the large aperture the shallower DOF

Also affected by zoom length, the longer zoom the shallower DOF

and the distance to the subject,  the closer the shallower DOF

example of thin DOF , if you want to emphrase the subject ,e.g. portrait;  look at the blurry background

use large aperture in these cases,to reduce DOF

 

full DOF , usually in landscape cases, the whole scene is perferably clear. use small aperture to maximize DOF