Some ILIAS maintenance actions can be run on a regular basis via cron jobs.
Examples are system checks (including purging older deleted files, database structures etc.) or importing users  from external sources like LDAP.
 
To activate these tasks, ILIAS provices a script cron/cron.php within the ILIAS installation directory. This script needs the credentials of an ILIAS administrator and the ID of the ILIAS client sytem (In case of multiple clients, this script needs to be called for each client individually).
 
So create a new user (usually in the local ILIAS database) and add a new job for cron.
Usually this job will be run on a daily basis, so create a new file with this content in /etc/cron.daily:

0 4 * * * www-data /usr/bin/php5 ILIAS-install-directory/cron/cron.php UserName Password Client_ID > /dev/null

Replace ILIAS-install-directory, UserName, Passowrd and Client_ID by your values. If your apache webserver uses a different user name than www-data change this value, too.
The first two entries determine minute and hour for the cron-job to run.
 
Some more explanation can be found at: Cron Jobs in another section of this module.