ISO (International Organization for Standardization) is the number that represents how sensitive your camera sensor is to light.
By having a lower ISO value it makes your cameras sensor less sensitive to light which is perfect for locations with lots of light. By having a High ISO value it makes your cameras sensor more sensitive to light which is ideal for locations with minimal amount of light. However using a High ISO value can affect the image quality of your photo making it appear noisy and grainy.
High ISO is sometimes needed because if the conditions of your photos are dark and doesn’t feature a lot of light then a higher ISO can help brighten the image but also degrading it in the process. Below is an example of an image taken from a Low ISO value at the start to a High ISO value at the end.