When you take a picture with a camera, light enters the lens and is focused onto the image sensor or film. The image sensor or film is then exposed to light for a certain amount of time, depending on the shutter speed. The amount of light that enters the camera is controlled by the aperture. The more light that enters the camera, the brighter the image will be.
Once the image sensor or film is exposed to light, it captures the image. The image can then be viewed on the camera's screen or transferred to a computer or other device.