Exposure!
The word exposure is very confusing to beginners in photography.
There is no single switch/buttons to set the exposure in the camera!
Yet, if you read a photography manual, most of the time it will talk
about setting the correct exposure!
Exposure means the amount of light that is allowed to fall on the film
through the lens aperture and shutter curtain. Therefore, if you want to
control the exposure you can do it either by varying the aperture of the lens or by varying the shutter speed or by varying both.
Variation of both these parameters affects different aspects of a picture.
For shallow depth of field as in a portrait, you need a large aperture;
whereas for large depth of field as required for a landscape photograph
you need a small aperture.
To photograph moving objects(action pictures), you will need a faster shutter
speed. For stationary objects, you will need a slower shutter speed.
Aperture settings are marked as fstops - you can vary the fstop number.
The larger the fstop number the smaller the apperture and so, more depth of
field.
The larger the shutter speed number the faster the shutter speed;
So, for a fast shutter speed, the shutter curtain will stay open for less
time than for a slower shutter speed.