Depth of Field or Focus relates to the area within the frame of your image that appears to be in focus (close to distant). It is controlled by a number of factors, the focal length of the lens, the aperture used, distance of the subject from the camera and  the size of the imaging medium (sensor or film).
A wide angle lens, that is one that has a small focal length will provide a naturally deep Depth of Field.
A small aperture will also give a greater area of focus depth than a large aperture.

To understand size of aperture it is necessary to understand the numbers used, which appear a little random. An aperture of f2.8 is larger than an aperture of f16. To remedy your understanding of aperture size, think of the numbers as a reciprocal with a 1 above; therefore 1/2.8 is larger than 1/16. As the Meercat says “simples”.
So for a given focal length lens f16 will give more depth of focus than f2.8.

If your camera is close to your subject, then depth of focus is less than if the subject was further away. This can be noticed with Macro (close up) lenses, where depth of field is virtually non-existent . This lack of deep focus as a result of magnification effect is also visible in telephoto lenses.

The size of the imaging sensor (digital or film) has an impact from the point of view that extremely short focal length lenses have to be used where the image sensor is small. This is why an image taken with a digital compact always seems to be sharp throughout the frame.

Extreme depth of focus isn’t always sought. By using a large aperture, the small area that is rendered in focus can pleasingly throw distractions in the background out of focus and thus draw total attention to the subject.

Hope this helps. Any suggestions on making this easier to convey would be appreciated.

More to follow.

Back
Deep Focus front to back
Wide Angle lens (18mm) and small aperture (f16)
Shallow Focus narrow strip
Macro lens (50mm) and large aperture (f2.8)