At the most basic level a camera is a device for letting light very briefly hit a piece of film or a digital sensor.
Photographers can adjust three things; the ISO, the shutter speed and the aperture.
The ISO is the sensitivity of the film or sensor. A low ISO is better suited to sunny or bright weather, while a higher ISO enables shooting in low light areas, even at night. However images with a very high ISO are very noisy, which appears as grain in film and ugly imperfections in digital.
The shutter speed is the length of time the shutter stays open. Long shutter exposures allow for blurring of movement, or if the camera is on a tripod allow for shooting in low light.
Henri Cartier Bresson- Behind the Gare St. Lazare 1932
The aperture is a set of blades within the lens which controls the amount of light going into the camera. Adjusting this will change the depth of field of the photograph, with a higher f number creating larger depth of field. Famously Ansel Adams the great landscape photographer named his photographers group f/64 hence GIANT depth of field.
Ansel Adams – The Tetons and the Snake River 1942
Every level of light has a recommended exposure to create a photograph and adjusting these settings around that exposure allows a photographer to take more creative decisions. This is why using a camera on a manual or semi manual setting is useful.