I would imagine it is the same as the flight path vector on the 777 and as fitted in most modern electronic flight decks.
The symbol shows where the present flight path will take the aircraft, a very very useful thing. So if you put the symbol on the horizon, the aircraft will fly level, reguardless of speed power nose att combination.
Similarly, if you put it 3 degrees below the horizon, you will descend at 3 degrees, reguardless of wind, speed etc.
Uses inertial and air data to figure it out. I personally use it all the time, even if just monitoring the A/P.
Stay safe and watch out for gravity