ISO aka ASA, basically known as film speed.

ISO determines how sensitive your sensor is to light.

-Higher ISO = Brighter Image will be = Lower Noise
-Lower ISO = Darker Image will be = Higher Noise

Noise is basically color grains = lower the fine details and color in your photos.

So basically when shooting in a low light situation/environment, simply use a tripod when possible.
Keep the ISO as low as possible using the Larger F stops or Shutter Speed.