ISO

What ISO denotes is how sensitive the image sensor is to the amount of light present. The higher the ISO, the more sensitive the image sensor and therefore the possibility to take pictures in low-light situations. 

And, where you would have needed to physically change to a different roll of film if you wanted a different ISO speed, digital technology allows you to simply dial one in. 

In this way, you can record images taken at different ISO speeds on the same memory card.

Suppose your digital camera's light meter warns you there is not enough light to correctly expose a scene. 

You could use the on-board flash, but let's suppose again it's not allowed (like in a concert or indoors recital).

You would then need to use a higher ISO. Set on "ISO Auto" mode, your digital camera will automatically select a higher ISO. 

Otherwise, you can manually select the next higher ISO and see if the increased sensitivity allows you to obtain a correctly exposed picture. If it does, you can now take a correctly exposed picture.