WebDAV (aka DAV) stands for "Web-based Distributed Authoring and Versioning". This functionality allows you to edit and save documents shared with other people on a DAV-enabled server.
See WebDAV Support for more details on the WebDAV standard support.
With Serna you can:
Open and edit documents from the DAV server
Save a document to DAV server
XInclude portions of the document from the DAV server
Reference external entities from the DAV server
Reference other resources (such as images) from the DAV or HTTP server