ISO

ISO is the level of sensitivity of the camera to available light. The lower the ISO number, the less sensitive it is to the light, while the higher the ISO number the more sensitive it is to the light. With the increased sensitivity, the camera sensor can capture images in low-light environments without using flash. However the higher sensitivity comes with a cost - it adds grain or noise to the video or pictures. 

An example is from this:

As we can see on the right we see a lot of noise than the one on the left. Every camera is has a base iso which is the lowest sensor that can produce the highest image quality without adding noise. Typicically we should stick to the base iso to achieve the best images however it is not possible to do when working under low-light environment.