Subsite settings let you override many of the default iCM-wide settings.
In many cases you'll want to use the defaults for all of the display, validator and content import settings. If you do, check the "Use defaults" checkbox!
If you do decide to create custom settings for your subsite, you can find information about the display settings and document import in the Content Settings article.
You'll find information about article validators in the Article Validators article.