Should I be afraid?

A little bit, it is one of the more serious issues that Apple has had with macOS. Anybody who has physical access to your Mac or can get to it through via screen sharing, VNC, or remote desktop can gain complete access to the machine.

Help me! What do I do to fix macOS?

The issue only affects High Sierra and Apple have already released an update. You can download it using the App Store app on your Mac, click on Updates and install Security Update 2017-001.

Opps, they did it again!

If you install the macOS High Sierra 10.13.1 update after installing the security patch you will need to install it again and restart.