WebMail Pro uses well-known, solid solution for storing contacts and calendars, DAV server SabreDAV. It's shipped as a part of WebMail Pro, so you don't need to install it separately. And aside from providing storage backend, it also allows for syncing data with mobile devices or any application/platform which offers CalDAV/CardDAV support.
For Apple clients to work, DAV must be on a separate domain (e.g.
dav.yourdomain.com) or port (standard value is 8008) where domain root immediately points to the DAV entry point. Apple clients don't understand suffixes after the domain name like
/dav.php/. Check DAV server configuration page for details.
With the feature enabled, you can log into email account with WebMail Pro to find all the sync settings within user account settings screen. Some devices/applications require DAV server URL while other need URL pointing to specific calendar or address book, and Mobile sync screen of account settings contains all these.
And in case of iOS-powered device like iPhone or iPad, you won't really need to deal with DAV URLs - just log into WebMail Pro from your device, and you'll be offered to get iOS profile which will automatically configure your device for syncing mails, contacts and calendars.