ISO (International Standards Origination) is a numeric value that specifies how sensitive the imaging chip is. As a rule-of-thumb we generally choose a low ISO setting because image quality gradually deteriorates as we go to higher and higher setting. If we choose a super high setting the image likely will show some granularity. This is akin to what is called “grain” in conventional film photography. The granularity seen in digital photography is called “noise”. It is analogous to static in an audio system. As the ISO is turned up, the image signal is amplified. This amplifies the good signal however immingled is some bad signal that also gets amplified. Modern digitals sport noise suppression so likely you will not see any ill effect except when the ISO is set very high.
As to exposure: Good exposure is the key to this kingdom. Too much exposure and the resulting image will be washed out (too light or even white). Too little exposure and the results are dark, perhaps even black.
Now the old box cameras of past (Kodak Browne and similar), sported no user settings. The camera was pre-set to a small aperture diameter and the shutter was set just fast enough to allow the camera to be hand-held. The focus distance is also pre-set for these cameras. Because the aperture setting is tiny (as to diameter), the zone of acceptable focus called depth-of-field is from about 4 feet to as far as the eye can see (infinity symbol ∞). Billions and billions of acceptable pictures were taken but picture taking opportunities were restricted to bright sunlit vistas.
To expand the picture taking opportunities to twilight or indoor etc. it is necessary to provide the user with options that allow more light into the camera during the exposure. Now we are talking, adjustable aperture diameter. Because the lens acts like a funnel, we can open up its diameter to allow more light to enter. This is a wonderful approach however; larger working diameters deliver a shallower zone of depth-of-field. Once we open up the aperture, we are forced to provide the user a focusing apparatus. In other words, adjustable aperture expands the range of picture taking but now we must focus the camera.
The old Brownie’s shutter was pre-set to about 1/25 of a second. At this shutter speed, lots of light plays on the film (now digital image sensor). Now you need to know that the exposing light accumulates all the time the shutter is open. A long exposure time accrues light. The downside is, we must hold the camera super steady and we must tell our subjects not to move. Once we provide an adjustable faster shutter, we need not be too concerned with camera of subject motion. This is a give and take. A fast shutter reduces light accumulation. The countermeasure is to open up the lens aperture. The added light gathering now affords a fast shutter.
What I am trying to tell you is: We have at our disposal, adjustments that allow us to expand the picture taking opportunity. We have a. ISO b. shutter speed c. aperture. We even have a fourth solution, we can supply artificial light. The idea is to expand the picture taking opportunity. The thing you need to know is, all of these adjustments are intertwined. We can adjusts just one or two at a time or all.
You need to study the mechanism of exposure. In the vascular of photography, this subject is called the “exposure triangle” a. ISO b. shutter speed c. aperture. If all this is too confusing, you need only to set your new camera on automatic. In this mode, the camera will make all these selections for you and the outcome will be OK. If you are serious about your new hobby, then you need to study the “exposure triangle”.