The Exposure Triangle

There are 3 main parts to getting the correct/desired exposure, typically referred to as the exposure triangle. Which is the Shutter speed, aperture and ISO. The relationship between these three things is what gives you your desired exposure regardless of whether you want it over, under exposed, or correctly exposed.

The shutter speed is the measurement of time that the shutter is actually open and exposing light on the sensor. Basically a curtain that comes down in front of the sensor and the speed is the time in which that sensor is exposed to light.

ISO is the sensitivity of the sensor or the film to light, so the higher the ISO the more sensitive the shot will be to light.

Aperture also known as the f-stop is just the size of the opening inside the lens. The larger the opening the  smaller the f number. Metaphorically it is like the iris of your eyes as the smaller the iris is in the light for a camera shot the number would be so f 16 is very small compared to 1.2 which makes the camera lens sensor more exposed to light.

The slower the shutter speed the sharper the image will be as it causes less motion blur. The faster the shutter speed the darker the image final result will be. The lower the ISO the less sensitive your shot will be but if this is still not bright enough you need to increase the ISO which can introduce noice and grain into the image. Then with the aperture ideally should be kept at the lowest number your lens can go g however the wider the aperture, the shallower the depth of field, making it hard to pull focus. This Is why it is important to work out the relationship between these three things. To customise this for more specialised images you have to shoot in manual mode instead of automatic.