Its purpose is to keep the computer current with the latest security (and other) updates automatically. If you plan to use it, you should have some means to monitor your systems, such as installing the apt-listchanges package and configuring it to send you emails about updates. And there is always /var/log/dpkg.log, or the files in /var/log/unattended-upgrades.
To install these packages, run the following command as root:
apt-get install unattended-upgrades apt-listchanges
To activate unattended-upgrades, you need to ensure that the apt configuration stub /etc/apt/apt.conf.d/50auto-upgrades contains at least the following lines:
APT::Periodic::Update-Package-Lists "1"; APT::Periodic::Unattended-Upgrade "1";
This file can be created manually or by running the following command as root:
dpkg-reconfigure -plow unattended-upgradessource: https://wiki.debian.org/UnattendedUpgrades