What is a cardDav account

CardDAV is a collection of standard formats and a network protocol that allows a client (iOS or OS X) to access contact information - names, addresses, telephone numbers - stored on a server. In an OS X configuration, this is the Address Book app, and the Address Book Server on OS X Server.

Depending on the particular context you're looking at, a "CardDAV account" is the credentials used to log in on and access the contact information on the remote CardDAV server.

Less likely to be the question here but still possible (given you're posting in the OS X Server forum, and OS X Server does have an Address Book Server), a "CardDAV account" can also be the account that the CardDAV server itself is running under.