When a Macintosh starts, it looks inside the folder "Startup Items" inside
the System folder and tries to open all of the icons it finds in that
folder. This is useful when you have applications, documents, desk
accessories, and control panels that you want to have opened automatically
at startup.
If you see the message "Some items could not be opened" you have probably
placed an icon that cannot be opened (like an INIT, system extension,
printer driver, and so on) inside your Startup Items folder.