Installation 00, or the Ark (therefore, not Halo, there is no Halo 00), is the control component of the Halo array, placed outside of our galaxy and therefore ouside the firing range of the array. It is a star-shaped (as in sea-star, not an actual star) artificial world about 80 000 km across, containing huge amounts of habitable surface on its "arms". It is capable of building other Halo installations.
You may also be referring to the 2D fan game, "Halo Zero". You can find more information on its Halopedia article.