A Requiem is a piece of music written for the dead.
Requiem aeternam dona eis Rest eternal grant unto them.
Now, here is the Latin prayer in English, I believe this may be a small clue to what this Forerunner place is:
> Eternal rest, grant unto them O Lord
> and let perpetual light shine upon them.
> May they rest in peace
> May their souls and the souls of all the faithful departed,
> through the mercy of God, rest in peace.
Now Halo 3 had religious references so I am going to assume Halo 4 may also.
“Requiem” may be the last Forerunner fort in the galaxy or a mass Forerunner graveyard with a possible population.
(And yes I am grasping at straws, but I like -Yoink- like this)