Depth of field.
Put simply, depth of field is all about sharp versus blurry images.
Below are two images of horses. In the first the horse is in focus, as is the wooden barrier, the carboard cut-out knight and even the wire fence in the background. It is only the distant trees that are out of focus.
In the second photograph the horse is again in focus, but behind him, the wire fence and tree are out of focus.
The difference in depth of field is caused by changing the aperture size. The apperture is the opening in the lens that lets light hit the sensor at the back of the camera. The largest aperture (usually f2.8) gives the shallowest depth of field. The smallest aperture (f22 and higher) will allow everything to remain in focus, but since the aperture is smaller, less light comes in, resulting in darker photos unless you compensate by slowing the shutter speed a little.
To increase the depth of field:
Select smaller apertures like ƒ/8 or ƒ/16
Move farther away from your subject.
To decrease the depth of field:
Select larger apertures like ƒ/3.2 or ƒ/4
Move closer to your subject.
We can use depth of field to force the viewer to look more closely at the in-focus elements of an image. For example, in the image below I opened up the aperture to create a shallow depth of field. I focused on the 'business end' of the beer bottle and clicked the shutter. With a wide depth of field the grass and rest of the bottle would also have been in focus and there would no longer a single item in the image to hold the viewer's attention.