ISO
ISO is the digital equipment version of ASA or the old 'Film Speed'.
It's how 'sensitive' to light the camera's sensor is. How 'magnetic' to light the bucket is. A high ISO in the camera means that the bucket of light will fill quicker, but, as in film, the higher the ISO the grainier the result. The sensor 'sparkles' more, degrading the photo, unless of course you want that effect.
I try not to adjust the ISO if I can get away without it, mainly because I forget to set it back to more normal levels afterwards and ruin up a whole day's worth of shots the next time I pick up the camera!!

​So, increasing the ISO means that you can take photos in less light with the same aperture and shutter speed as you would if there were more light.