Question about bootup

[Bart Silverstrim]

I have a relative with an older Mac (running MacOS, not OSX) and when 
they try to boot up the system it "locks" with a dialog box that is blank.

If they reboot and hold "apple-shift-control", the system boots up and 
gives some errors about open transport not starting and some quicktime 
server extensions not working because open transport isn't loaded up.

What exactly does that key combination do?  At first I thought they were 
disabling extensions, but the startup icons for extensions appear one by 
one despite those keys being held down.

Any help here?  Googling for keyboard shortcuts now keeps giving me 
references to OS X and I don't see the key combination in the lists that 
I wrote above.