The ability of the camera lens to vary the amount of the scene that appears in focus, depth of field, is one of the most useful creative tools available to the photographer. Say for example you were taking a head and shoulders portrait of a friend in their back yard, unfortunately, their yard leaves a lot to be desired, in terms of attractive backgrounds for the portrait. But with the depth of field control of your lens you can over come these difficulties, by choosing a shallow depth of field, the resulting picture should result in only your friend appearing in focus with the unattractive background no more than an indistinct blur. In other situations it may be desirable to have as much of the scene in focus as possible, such as photographing family members on holidays, so people who are looking at the photograph will be able to see clearly where you and your family were. So it is always important to consider the appropriate depth of field required before taking each photograph.
Aperture is critical to depth of field.
The aperture of the lens is usually controlled via the ring closest to the camera body. The aperture itself is an adjustable diaphragm, a series of overlapping metal blades that operate together much like the iris of the eye. The numbers on the ring refer to a particular aperture, the numbers themselves are actually fractions which refer to the size of the aperture relative to the size of the lens. Apertures work in factors of 2 with each other, the apertures each side of your chosen aperture either admits twice as much or half as much as your chosen aperture, so f11 allows twice as much light into the camera as f16 and f22 allows only half as much light to reach the film as f16. The amount of light that a particular aperture admits is consistent no matter what lens is used with your camera, which makes life easier in situations where you have calculated your exposure and you then decide to change lenses.