What is Fast Startup?

Fast Startup makes boot quicker by saving a mini-hibernation image of the Windows kernel at shutdown instead of fully closing everything. Most of the time you'd never notice it exists.

It matters when troubleshooting: with it on, "shut down" isn't a true fresh start — but Restart is. That's why restart fixes problems shutdown doesn't. Disable it via Control Panel → Power Options if dual-booting or chasing odd hardware bugs.