It's what it says: Num Lock. fn+F6 toggles it. When active, the "embedded" number pad whose key labels are shown smaller than the main key labels and cover the oblique "rectangle" of keys from 7-8-9-0 on the top to M-,-.-/ on the bottom.
Randall Schulz
iMac 20" Core Duo; MacBook Pro Mac OS X (10.4.6)