The Eclectic Light Company:

Upgrading to High Sierra brings a new and significant security feature: your Mac will automatically check its EFI firmware. In a series of tweets, Xeno Kovah, one of the three engineers responsible for the new tool, has outlined how this works.

The new utility eficheck, located in /usr/libexec/firmwarecheckers/eficheck, runs automatically once a week. It checks that Mac’s firmware against Apple’s database of what is known to be good. If it passes, you will see nothing of this, but if there are discrepancies, you will be invited to send a report to Apple.

And:

eficheck depends on a small local library of ‘known good’ data, which will be automatically and silently updated if you have security updates turned on in the App Store pane.

That checkbox is in the App Store pane in System Preferences and should be checked by default.