It enables the daemon to use the common tcpwrappers access control library while in standalone mode, and in a very configurable manner. It is not compiled by default.
If not installed on your system, the TCP wrappers library, required by this module, can be found here, on Wietse Venema's site. Once installed, it highly recommended that the hosts_access(3) and hosts_access(5) man pages be read and understood.
Many programs will automatically add entries in the common allow/deny files, and use of this module will allow a ProFTPD daemon running in standalone mode to adapt as these entries are added. The portsentry program does this, for example: when illegal access is attempted, it will add hosts to the /etc/hosts.deny file.