ISO is one of the three pillars of photography, the others being aperture and shutter speed. These three make up the basics to be able to use your camera out of auto mode and be a lot more flexible with your photography, so understanding them is key to taking your photos to the next level.
As simply as possible, ISO is how sensitive to light the sensor in your camera is, read on for a more in-depth description.
So, what is ISO and why should you use it?
As we have said above, ISO is how sensitive to light your camera's image sensor is. The higher the ISO number, the more sensitive to light the sensor is, allowing you to take pictures in increasingly low light - but this comes at the price of adding grain/noise to your image.
You should always try to keep your ISO as low as possible for the best quality image without any noise. But when you are shooting in darker conditions and want to keep your shutter speed up, then you can increase the ISO to allow you to do this.
The below image was taken in a small candle lit Estonian pub, it was fairly dark. If you look closely you can see the grain from the high ISO I used to freeze the motion.
ISO is a pretty easy pillar to understand because what it effects is very clear. Go and try using ISO in some low light conditions and see how the shutter speed can be increased so you won't have blurry images.