Depth of Field (also known as DOF) is an optical phenomenon related to focus. For a beginner's eyes, a photograph can be in focus or out of focus (when it looks blurred), but in a more advanced way, this is not absolutely true.
It's important to understand that in the real world, a scene will show some parts in focus while others will be out of focus. There will be a certain distance to the camera where things look nice and sharp. We call this point the focal distance.
The focal distance is crucial when applying the DOF effect, because this will set the exact point from where things will start to get blurred the more they move away. In other words, the further you get from the focal distance, the more blur you get.
In the following image, you can see two examples of DOF using a different focal distance. The image on the left shows a short focal distance—that's near to the camera—while in the right-hand image, it's moved far away from the camera, showing different focal...