The briefcase is a piece of equipment available in Condition Zero: Deleted Scenes that is used to send a signal to friendly units.
It can only be used in Briefcase zones. Inside the briefcase, it contains a lot of (possibly fake) U.S. Dollars and it is also equipped with the radio transmitter that will signal your teammates, thus starting the assault.
Once you reach a Briefcase zone, the briefcase icon will appear. Select your briefcase, and press your primary fire (MOUSE 1 by default). The player will open the briefcase, revealing the money.