This is for what is displayed at your site’s main URL.
Depends on the site’s “Front page displays” Reading Settings ‘show_on_front’ and ‘page_on_front’.
If you set a static page for the front page of your site, this function will return true when viewing that page.
Otherwise the same as is_home().
For more information on this and similar theme functions, check out the Conditional Tags article in the Theme Developer Handbook.