The “applewebdata” URI scheme is used by iOS and Mac OS X instead of the “http” in URIs used for accessing internal resources on the device or computer itself. The visitors likely came to your site whilst using their iOS or Mac OS X device, being refered there by documents stored on the device rather than from a web page.
The referrer is determined from the headers sent by the browser that visits your site. By convention, this should contain the URL that referred the visitor to your site - but it could be set to absolutely anything.
You may be able to gain more insight by seeing which browser tag accompanies this referrer.