Along with aperture and shutter speed, ISO is one of the three fundamental exposure parameters, and mastering it is crucial to being able to produce successful images. If you are newer to photography, this helpful video tutorial will show you what ISO does and why photographers sometimes use high ISOs.
Coming to you from David Bergman with Adorama TV, this great video tutorial will show you some important concepts about ISO, particularly high ISO values. A common mistake newer photographers make is sacrificing shutter speed in an attempt to keep ISO lower. It is good to try to keep the ISO as low as possible, as this gives you less noise and greater dynamic range, but if you make your shutter speed too slow, you risk introducing blur either through camera shake or subject motion blur. The important thing to remember is that there are always techniques to deal with noise from a high ISO in post, but generally, once a photo has blurring from camera shake or the motion in it, there is not much you can do to save it. It's always better to have a noisy, sharp photo than a blurry image. Check out the video above for the full rundown from Bergman.