ISO ( digital camera sensitivity)
The ISO setting on your DSLR adjusts how sensitive your camera is to light. The lower the ISO number the less sensitive the camera is to light. The higher the number, the more sensitive it is to light. So for a shot outdoors with good sunlight you would set your ISO as low as 100. If your inside a place like a museum where flash is not an option, you would raise your ISO to 1600 or higher. You will learn by experience what ISO number is best for your various locations.
As always, every digital camera has a variety of automatic modes that allow you to have the camera set the ISO for you (Automatic, Auto-no flash, Program, Aperture Priority, Shutter Priority etc). You can find the details of each setting for your specific camera in the included camera manual.
In case you were curious (as I was),ISO stands for: International Standtards Organisation, it refers to the industry norm for sensitivity of emulsion based film.
Now that we got that out the way. The ISO setting rounds out the three foundational pillars of photography (Aperture, Shutter Speed, ISO). Adjusting your ISO impacts how fast you can set your shutter and how wide you can set your Aperture. They all work together.
Have any questions or something to add, leave a comment!