Hi! This machine has an open firmware password enabled, which is similar to a BIOS password on a PC. This is not the same as the admin password, i.e. it's in the firmware of the machine itself, and not something on the hard drive.
Basically, this means that a previous owner of the machine decided to lock it, and this lock will prevent you from booting to other drives, optical media, etc. If you know the password, you can go into open firmware and disable the lock. Otherwise, there is a procedure to override it: You need to change the amount of RAM in the machine (either add memory or remove existing memory in the expansion slot), and IMMEDIATELY, i.e. the first time booting after changing the RAM, reset the PRAM three times in a row. That means it needs to chime three times after the power-on chime. Once you've done this, you should no longer see the lock. It's Apple's "top secret" way to bypass an open firmware password.