depth of field

Depth of field is the range of distance around the focal point, which is sharp. The depth of field can vary depending on 3 things, camera type, aperture and focusing distance. Depth of field is usually when the objects closer to the point of focus are sharp, and there is a gradual transition to unsharp objects toward the back or in front of that object.
The diagram above shows how much a point needs to be blurred in order to be thought of as unsharp. The image below shows how the focal point is sharp, but each point afterward gets blurry. (Shown by how gray the circle gets)
So, objects farther away from the focal point become more and more blurry and out of focus so that the viewer of the picture can focus on the main part, without the distraction of the background. The depth of field can be set by using F-stops. The higher the f-stop, the less blurry the background will be. While a higher f-stop will make the focal point stick out even more.