A visit is one visitor's page views on one day. There are no cookies, so the visitor is a daily hash that is never linked across days: a visit can't span midnight, and the counts are estimates. A visit's source is the referrer of its first page; "Direct" means the browser sent none (typed URLs, bookmarks, many apps, and links from https pages to http). "Internal" visits came from one of your own pages: they began before the range you picked, or on a host that isn't listed in the site's settings.