If you’re using a modern DSLR, you don’t have to worry about ISO nearly as much as you would if you were shooting with a little P&S with a 1/2.3-inch sensor. I used to have an old Canon Powershot S3 that had visible noise at anything over ISO 100!
Here’s some reading material from Nikon itself on Auto ISO.
There’s also a post here with screenshots showing the different settings…