If you are using or have used an analog camera with “real film” you might have seen that film is sold with different ISO-values. This value tells us how sensitive the film is to light and by photograph using film of different ISO we can manage to get photographs in very low light as well as photos in a sunny day. In digital cameras the same value can be set telling the sensor inside the camera how sensitive it should be to light.
The most used span of ISO today is between ISO 100 and ISO 800. At the lowest value we need the most amount of light to get a good photo. When we increase the ISO value it mean that we can take the same photo as before but using less light.
The downside of a higher ISO is that the higher the value we use, the more blur we introduce into the photo. The real pros of using a higher ISO is that we can use a faster shutter even though the light is low. This is very useful if you are to take a photo in low light and have no tripod, by boosting the ISO you can keep the shutter fast and still get a good photo.
A rule of thumb is to keep the ISO at the lowest possible setting for the situation. In other words, start out by trying to take the photo using a low ISO and increase it until you get the outcome you were looking for.