What does "unable to verify startup disk" mean?

I've been using a Sequoia system on an external drive for some time, but today when I try to set it as the startup disk I get "unable to verify startup disk." If I I use startup options and select the drive, it seems to boot. The white bar gets to almost the end, then it sits there for many minutes. Finally it puts up "unable to verify the startup disk." What does this even mean? Who is verifying what? Who gets to verify the startup disk? What does it need to "verify" the disk?

I'm on a 2021 M1 Max MBP.