ISO stands for "International Organization for Standardization"
This term generally gets shorted to ISO which applies the sensitivity of the cameras sensor to light. On most professional cameras ISO can be low as 50 with an upper expanded limit of 204,800. The camera I have the Sony Alpha 7 IV has an ISO range of 100-51,200, with an expanded range of ISO 50-204,800. On most newer model phones the smaller camera sensor has a ISO range of 50-3200 which is more than enough.
Referring to my previous post the exposure triangle is very relevant in exposing an image. Light is measured in stops A stop of light is not a fundamental unit, but a method of measuring the increase or decrease of light in a photographic exposure.
When you increase the ISO value by one step (e.g. 100 to 200, or 800 to 1600) you make the camera’s sensor twice as sensitive to light. To keep the exposure balanced the camera needs to halve the amount of light reaching the sensor. It does this by increasing the shutter speed by one stop (e.g. increasing from 1/400 to 1/800), or decreasing the aperture size by one f-stop (e.g. changing from f/8 to f/11).
Once you begin to increase the ISO past "3200" on most professional cameras you introduce noise to an image. Now this noise is not referring to sound so your photo will not have a music track playing but it will display grainy spots to the image. Some photographers like this grainy noise on their images and keep it visible while others use Adobe Lightroom to remove it. Don't be afraid of pumping the ISO for those low light shots because the shutter speed and aperture will adjust to compensate.
