ISO stands for International Standards Organization. ISO can ranges in value from 25 to 6400 and beyond.
These numbers indicates how sensitive your camera is to light. The lower the ISO number, the less sensitive your camera's image sensor is and therefore the smoother the image, because there is less digital noise in the image.
The higher the ISO number (the more sensitive) the stronger the image sensor has to work to establish an effective image, which thereby produces more digital noise.
By bumping up your ISO (3200, 6400) you are allowing your camera to see in the dark. The only trade off is the quality of your photograph.