RE: ISO - it can be thought of as the sensitivity of the sensor. The higher the number, the more sensitive the sensor is to light (technically inaccurate - but close enough).
Each doubling in ISO is equal to the same effect as if you increased exposure by one "stop", i.e. twice as long shutter speed or one full "stop" of aperture. Aperture used to be controlled directly on the lens, each "stop" being represented by a detent in the aperture ring.
Today aperture is controlled by the camera, and isn't quite as clearly represented. If you have a f4 lens, then one less light would be 5.6, two stop 8.0. The scale is kind of hard to understand, but is equivalent to two or three (depending on settings) "clicks" on the control dial.
But everything is rational, meaning that one can compensate the other. So you can increase ISO to get shorter shutter speed, or more DOF from a smaller aperture, or you can have a longer exposure with a smaller ISO value to decease image noise. It all sounds SUPER complex, and you don't really need to "get it" right away. Just keep learning and shooting paying attention to what the numbers are doing, and it'll make sense.