It enables automatic security updates. You could also enable automatic updates for all, not just security. Basically have the system run the meme commands for you.
That's interesting, I didn't know this can be configured in one line. When I searched how to configure unattended-upgrades myself I only found long solutions.
I'm sure there are longer solutions as well. This is straight up from the Ubuntu wiki. If you want to configure it differently, e.g. do all updates, not just security, you probably have to change some more vars in the apt config files.
That's odd. If unattended upgrades are running, the system will do upgrades regularly. That means it's unlikely to get a significant backlog of updates queued up. Upgrade cycles typically finish briefly as a result. All my systems, interactive or headless, are running an update and upgrade cycle every hour. I've yet to to run into a case when I couldn't install a package because apt was in use. It's not impossible, but I haven't. Or at least it's been so long ago that I've forgotten about it. I don't have to think about unpatched vulnerabilities. ☺️