ISO– ISO was originally used to refer to the sensitivity of film, but is determined by the sensor of the camera for digital cameras & smartphones. Choosing the appropriate ISO is usually in relation to the overall lighting conditions of your scene. 

Different sensitivities of film were required for different conditions; daylight 100-200, indoors around 800. Digital ISO is in reference to the film standards above, but now span a wider range with some as low as 50 or as high as 204,800, though standard ISO ranges on most cameras are around 100-6400. It works by amplifying the digital signal generated from light hitting the sensor, and affects the quality, contrast & noise of the image. 

In the digital world, ISO is more akin to Gain in the sound world. If you have a quiet sound you wish to record, you up the gain so it is audible, but the consequence is a background hiss and a reduction in sound quality. Translated to the visual world, if you have a low light condition that you need to capture, you can up the ISO as much as is needed but the sacrifice is visual ‘noise’. 

Lower ISOs are suitable for bright light conditions, produce higher contrast images, and generally higher quality.

Higher ISOs are suitable for low light conditions, product lower contrast images, and produce more noise / grain. 

Image quality is sustained by utilising the lowest ISO possible for the light levels of the scene. 

ISO - Noise and Contrast