autodns-dhcp_cron is a cron script to check for changes to the dhcp.leases file, it calls autodns-dhcp_ddns whenever there are changes, so that the changes are updated on the dns server and also settled on the dhcp configuration. Also, it reloads dhcp server configuration whenever it has been changed. This is done periodically from the cron as stated on the cron file for the package, however it can also be called by hand whenever the administrator may want to.