A Grid is a Forge Item first introduced in Sandbox of Halo 3, then also available in Forge World of Halo: Reach, and forge maps of Halo 4. It looks like a blue coordinate grid composed of 9 large squares, each with 25 smaller squares inside them.
Grids can be used as alternative ground, such as on elevated maps. Players can walk on it, forge on it, and use it like any other large flat piece (such as a coliseum wall ), with the advantage of it looking futuristic as well as being transparent - which can change gameplay in maps with multiple levels. It can be very handy for people who are forging a 'floating' map or people who want symmetry. It is one of the largest single forge items in the Halo games, with enough space for a small 2 - 4 player map to be built upon it.
The Grid on Sandbox is not a forge item that can be deleted or moved - it is rather a massive forging floor high above the map. It is there to stop forge items falling to the ground of the map, which allows people to easily make maps in the sky (this was useful in Halo 3 since it was before the phasing feature was introduced), although players may not stand on it. It is only visible when in forge mode. If one goes into a custom game then it will appear to be empty open space, however, its effects still apply. If you are modding, you can also create Grids similar to the ones in in Halo: Reach.
The Grid makes a return in Halo: Reach and is now a forge item that can be created from the items list. The Grid costs $100. The surface of the Grid in Halo: Reach is seemingly made of energy and will react like one of the shield walls if shot at. It is also possible to lag (near crash) by placing a grid and phasing it into the ground then setting the grid to fixed or normal on any map. It is also possible to do this in walls or ceilings. The grid can also be used to make water in the oceans on Forge World shallower - by bringing the grid just a couple of feet below the water surface, it is invisible yet people can walk on the water without being killed.
The Grid returns in Halo 4's Forge and looks and behaves similarly to its Halo: Reach counterpart, although there may be some very slight aesthetic adjustments.