This will zero out the Master Boot Record/Partition Table.
It can be useful in cases where you have a corrupted
MBR and you want to start from scratch with a fresh
empty sector(for instance, if you had a virus in there
that couldn't be cleared by FDISK/MBR)

He shouldn't have given it to you without a good
explanation and instructions though. It will not
test your drive, it will effectively erase it.

http://www.firmware.com/support/bios/hdclear.htm