The depth of field is the range of the scene that appears sharp, commonly said to be “in Focus”. It is determined by the Focal Length and the Aperture diameter.
For landscape photos, the depth of field has to be huge (kilometers). For portrait photos, the depth of field might be just a few centimeters. The ideal depth of field depends on what the subject of the photo is and where/if it’s desirable to isolate the rest of the image.
A specific depth of field is useful for shooting through or past things - for example, a dusty window or even a dirty camera. Placing the window out of the depth of field reduces the impact of the dust on the image.