This is a bit trickier as when I was learning it didn’t make much sense to me and I couldn’t make head nor tail of what difference changing ISO would make. Essentially, ISO is a measure of how sensitive a camera is to light. So what? I hear you say.
Light comes into the camera down the lens, through the aperture, the shutter opens / closes and light hits the camera sensor. By changing the ISO setting you can change how sensitive the sensor is to light – not very much or quite a lot.
Small ISO numbers mean the sensor is not very sensitive to light and higher numbers mean the inverse (obviously) – more sensitive to light. This has the effect of changing the brightness or darkness of your image. Low ISO = less light so darker, high ISO = more light so brighter image. But, there is a trade off … higher ISO can make images look grainy and lose some quality…what’s known as ‘noise’ can creep into it. Think of old TV’s when they are not tuned into a TV channel and that speckly stuff shows…nothing that severe appears on images but something very faintly like that. Generally, low ISO gives sharper images.
ISO is written as a number – so ISO100, ISO 200, ISO 6,400 etc. Think of it as the number of receptors on the camera sensor that are waiting to receive light to form the image. ISO100 has 100 of them ready (and all the rest turned off – sort off); change the ISO to ISO6,400 and there are 6,400 of them capturing the same light so the sensor gathers more and the image is brighter. It’s not quite that but it’s something along the way to illustrate the purpose.
So that’s it – there are the three things that can effect an images exposure and hopefully you can start to see how changing one can affect the other… this is known as the EXPOSURE TRIANGLE.