Teleportation grid

The Teleportation Grid is a device invented by the Forerunner.

Halo: Combat Evolved
Forerunners have created this interesting piece of equipment to teleport from place to place. Sometimes you appear in another green teleportation pad, which is called a 2-way pad, or sometimes you land on the ground. This is called a One-way teleportation pad. Covenant forces have been seen coming in and out, and the MJOLNIR armor allows the bearer of the armor to pass through. Little else is known about this, and no one knows if anything but a Spartan and Covenant forces can pass, as only the Covenant and Master Chief have done so.

In Halo 2
The teleportation grid is portrayed very differently, as an instrument that covers the entire Halo installation and can be used by Monitors to teleport themselves and Reclaimers anywhere on Halo in a mere second. It at first seemed that only 343 Guilty Spark had access to such an instrument, but it was soon found that Gravemind was also able to use it, possibly through possession of 2401 Penitent Tangent, the monitor of Installation 05. Because Gravemind does not transport himself through the grid, it is possible that the armor of the Arbiter and the Master Chief contain some neccesary component Monitors have or the Flood are unable to teleport to prevent easy spreading.