Depth of field is something every photographer has to learn and gets to play with every time they push the shutter button. The most basic definition of depth of field I can give is the portion of the photograph in focus. If you have a large depth of field most if not all of the photograph is and focus and conversely if you have a shallow or small depth of field very little of the photograph is in focus.
Several factors come into play when depth of field is in question. Two of the biggest factors is lens size and aperture. Bigger lenses just inherently throw a smaller depth of field so you have to compensate for it in other ways if you want more of your shot in focus.
Here is a good example of this concept from a shot I recently took with two blue winged teal. I started photographing the lone back bird but another teal quickly moved into the shot and I didn’t have time to change my camera settings so I just kept shooting. As you can see the back bird is in focus and the front bird is a victim of a shallow depth of field, a much smaller dept of field than I would have shot this image with if I had two cooperating birds in the beginning instead of one cooperating duck and and one seemingly enjoying photo bombing my shot.
The interesting thing here is I was shooting this with a 600 mm lens which naturally shortened up my depth of field quite a bit, but which I like when photographing lone birds so the background is blurred and not too distracting. But when a second subject enters the field it changes everything as you can see.
The cool thing about depth of field and what is shown by this shot is it can highlight a particular part of the shot giving it more emphasis and strength in the photo. I personally would have loved to have gotten both birds in the shot clear and sharp but sometimes you have to run with what mother nature gives you and enjoy the results even if they aren’t what you first sought out to shoot.