Your cameras ISO setting determines how sensitive the sensor is to light. Normally the ISO range looks something like this 100 200 400 800 1600 3200 and so on. The higher the number the more sensitive the camera is to light. Each step doubles the sensitivity, for example ISO 200 is twice as sensitive as ISO 100.

 

So if you do not have enough light to shoot the picture you want, you can try and increase the ISO sensitivity to get a shorter shutter speed. However there is a catch. When you  increase your ISO speed you will also add more noise to your pictures. The reason for this is when you make your sensor more sensitive, the camera has to amplify the electrical signal from the sensor and the result is that both the image information and noise will be amplified.