Depth of field
Depth of field is how much the image is in focus, if it is soft and unfocused that is called shallow depth of field but if it is a landscape that has more of the image in focus that is called large depth of field. In every picture there is a certain area of your image in front of, and behind the subject that will appear in focus. An aperture F/2.8 is wide while an aperture of F/22 is small. A camera can only focus its lens at a single point that still appears sharp. Depth of field links to aperture, aperture is like a 'curtain' in the camera lens which allows you to control how much light gets let in. Also its like your eyes, when there is too much light pointing towards your eyes, your eyes shut slightly.