This is the default method of authentication. It stores usernames and passwords in the SASL secrets file sasldb. In this HOWTO, I assume you are using the sasldb method of authentication.
The following definition came from the OpenLDAP website. Lightweight Directory Access Protocol (LDAP) is an open-standard protocol for accessing information services. The protocol runs over Internet transport protocols, such as TCP, and can be used to access stand-alone directory servers or X.500 directories. This method isn't discussed in this HOWTO, but here is a link to a page with patches and info: http://www.openldap.org/faq/data/cache/428.html.