The term exposure in photography stands for a combination of Aperture, shutter speed and ISO. The image that your camera will produce after adjusting these three pillars of photography is what your final exposure is.
Aperture contributes to an exposure by controlling the amount of light that hits the sensor, Shutter speed contributes by controlling the length of time that the light will flow through the aperture and ISO contributes by either increasing or reducing the image sensor’s sensitivity to this light. Heart of any camera however and the factor that gets the aperture, shutter and ISO to work together is the lighting meter.
A lighting meter is a device designed to react to the light source and do the required calculations to determine if the exposure is correct or not. When I say correct exposure, the meter determines If the image will be too bright (over-exposed) or will be too dark (under-exposed). It takes a close look at the current Aperture and Shutter setting, do the math and then suggest if you need to change either of the settings before clicking the shot. For example, if you are shooting in dim light with a narrow aperture, the meter will calculate and may suggest you to slow down your shutter in order to give ample time to the sensor to gather right amount of light.
For better standing, Imagine an empty tank of water, a faucet, and 100 workers with empty buckets. Job of the workers is to take water from the faucet in their respective buckets and dump it in the water tank in order to fill it up. Now the water tank is the image sensor, faucet is the aperture opening, workers are your ISO setting and shutter speed will be the amount of time the faucet will remain open. The lighting meter considers all these factors and based on its calculations indicates if either of the setting needs to be changed. For example, if you select a certain shutter speed and leave the ISO setting at 100, the meter will calculate if the faucet opening is enough for 100 workers to get ample amount of water in chosen time for the tank to be completely filled. If it isn’t, then it may suggest you to open the faucet a little more so that the flow of water can increase, to widen the aperture. Same way, If you select a desired aperture at ISO 100, the meter may indicate that you need to increase or decrease the time frame. For another example, let us assume that you have a wide open aperture and have slowed the shutter down to as much as you could but the meter still indicates that the image is under-exposed, that the water tank can still not be filled. So what do you do in that case? You bump up your ISO to 200 meaning that you deploy more workers on the job.
Let us say that you were photographing a flower. You want the flower to stand out and blur the entire background. You camera however indicates that it will use an aperture setting of f16 and a corresponding shutter speed. This will produce a correct exposure however an aperture that narrow will bring the entire background in focus. Everything in the picture will stay sharp and in focus which will beat the idea of keeping the focus just on the flower. To rectify this, you put your camera in a manual mode, widen the aperture that will produce the desired effect and then start changing your shutter speed until the camera meter indicates a correct exposure.