Multiple Threat Alert Center or "MTAC" is a room in the NCIS building which is used by various members of the NCIS agency with all members often using it to keep a process on the events going on in the world or stage conference calls with agents from other agencies including the CIA, FBI, DEA and even members of the many NCIS teams stationed around the world and around the United States with the prime examples being conference calls with the teams in the NCIS offices in Los Angeles and New Orleans, respectively.
Numerous conference calls have been also had, even with those currently active in the United States Navy and Marine Corps as well as members of other agencies including Interpol and the State Department.
The room is huge and has live computer monitors as well as TVs on the walls as well as a large screen in the center of the room.
In addition, two techs are present at all times at the monitor stations and will often inform whoever's in the room of an incoming call.
Presumably for various security reasons, access to MTAC can only be granted to members of NCIS whose patterns have been recognized by an iris scanner located outside the entrance to MTAC.
Once they've been scanned and cleared, the person in question will be granted access to MTAC.