"Direct Traffic" (afak) basically refers to all traffic that doesn't have the "Referer" header set. This is generally the case when you type a URL directly into your browser (or paste it in), click a bookmark, get directed to a page from a different application (for example: instant messenger or text)
That said, often the "Referer" tag is explicitly hosed. For example when you click a link in an email, instead of showing up as traffic from "mail.google.com" you're shown as direct traffic. Unfortunately for analytics (again afaik), there are a number of ways/sites that deliberately hose the Referer tag for whatever reason.
Because of this, the "Direct Traffic" section is often larger than you'd hope or expect for just email users. A portion of those users are definitely traffic from emails, and may of them will be people sharing links directly with their peers.. but some of them will be simply sites that are masking their Referer.
Unfortunately, Kickstarter's analytics isn't that great.. the normal way to overcome this limitation in tracking is to append to the URL a special tag that is only used for analytics. For example..
http://www.example.com/some/page?source=emailWhen someone clicks that link, it'll go to the same place as all other /some/page links go.. but your tracking system will read the "source" value and count the traffic accordingly.