An invisible wall is a term for a boundary that limits players to a certain area. Essentially solid walls that cannot be seen, they are typically used to prevent players from exiting a level's boundaries or entering areas that they are not supposed to access. They tend to be used as a catch-all on the off chance that a player can scale huge cliff faces or make it past vast bottomless pits.
In the Halo games, invisible walls are often supplemented with kill barriers. Kill barriers, also known to some as death planes, are similarly invisible, but instead of blocking the player's movement, they simply kill the player on contact. Such kills are usually marked with the phrases "You fell to your death." or "You were killed by The Guardians." If, while in Halo 3's Forge, a player manages to place a vehicle outside of a map, amusing deaths can result when the player tries to enter any seat of that vehicle that just happens to intersect a kill barrier.
There are also some kill barriers that behave strangely. These kill barriers do not instantaneously kill players; instead, there is a delay between the bypassing of the barrier and the death of the player. It is possible that these barriers are meant to supplement "unstable" walls that may be partially bypassed by players without them completely leaving the map. Such a kill barrier can be found beyond the fence in Halo 3's map The Pit, where players in Player Mode will die if they stay beyond the fence for more than roughly five consecutive seconds. Players in Edit Mode seem to be immune to these barriers.
Even when kill barriers are not present, there is often little point in bypassing invisible walls -- often, the terrain beyond them lacks collision data, causing anyone who bypasses the walls to fall through the ground on the other side and to their death. Several walls in Halo 3 have the odd quality of being solid on both sides -- invisible walls in most games tend to only be solid from one direction, as a result of backface culling.
Halo 3 Specifics
Halo 3 uses several common techniques. In addition to normal invisible walls, most maps in the game have invisible ceilings -- a single flat plane placed above the entire level, blocking all movement. Some maps use "repelling walls" -- walls that not only block the player's movement, but also push the player away. An example of such a wall can be found in the Multiplayer map Last Resort, by attempting to fly on top of the large structure above the "Camp Froman" area.
In Halo 3, some invisible walls, including the invisible ceiling on the Multiplayer map Ghost Town, can be bypassed by corpses. When editing map variants in Forge, it is possible to spawn items and forcibly push them past invisible boundaries, though such items are nearly always deleted once the editor "lets go" of them. A glitch allows you to work around this limitation, though there is often little point in doing so -- Teleporters and Respawn Points placed with such a method would only cause instant deaths, and other items placed beyond such boundaries would be inaccessible.
Use in Glitches
Often, invisible walls may only be bypassed by using glitches. Many people deliberately try to bypass such barriers as a hobby, typically using glitches in Halo 3's Forge to achieve the desired result. Many videos of such glitches can be found on YouTube.
In Halo 2, the Energy Sword lunge contained a glitch that made it a very practical way of getting past invisible walls. With one player on the other side of the wall, you can lunge/cancel on him until you go through. If there is a height limit on it, such as the invisible wall on the top of Gemini, you can lunge upwards and over the wall by being farther below the "Lunge Host".
Sword lunging is not the quickest way of getting past an invisible wall, but it takes less know-how to accomplish. It is not guaranteed to work on all invisible walls.
Super Jumping is the general term used to refer to any method of jumping abnormally high. Super jumps make it possible to get to higher areas within levels and places that are not supposed to be accessed without a Banshee -- or at all. They are useful for bypassing invisible walls that Banshees cannot pass through. The super jump on the Halo 2 Multiplayer map Ascension is a good example of getting past an invisible barrier; although the two person Banshee trick will work, it is easier to do the super jump during matchmaking.
The Explosive Jump is a type of super jump, as is the previously-mentioned Sword Flying technique.
Super Jumps can take time to learn, and are not always easy to accomplish on first tries. They can be tricky, making the player go through several elaborate tasks before being done. Jumping on different ledges and using perfect aim and timing.
Although it is easier to get over most invisible walls with the super jump, it is harder than the sword lunge, unless you know what you're doing.
On the first zero-gravity part of the Halo 2 Campaign level Cairo Station, if you manage to get to the airlock furthest from view from when you leave the first airlock, you will notice that the In Amber Clad is not very far away. If you jump from the furthest point reachable on the airlock ceiling, it is likely you would have crossed the wall, which on the entire level is only several inches tall.
Overload and Push
When on Forge with another player, you can exit the map. First, you need to overload it. Once that has been accomplished, spawn a turreted vehicle (Warthog, Scorpion, etc.), a Machine Gun Turret, or a Plasma Cannon. Allow the second player to enter the turret. Pick up the vehicle/turret and turn it so that the player is between it and the invisible barrier. Push them through. (If they do not pass through, turn them away from the barrier, push it in and drop it; then pick it back up and resume pushing.) When they are past the barrier, they can stay past it by dismounting; at this point, the player in Edit Mode may do whatever they wish with the turret/vehicle.
When viewing a saved Film in Halo 3's Theater, it is possible to glitch the camera beyond the map using the Pan Cam code. To do this in a Multiplayer map, simply play that map on Forge, and get your character to die while in Edit Mode; then, load up the saved Film, pause when your character dies, and accelerate to clip past the wall. The process is very similar in a Campaign map, but it requires that the player die by falling into a bottomless pit or some other large depression with non-solid/no terrain at the bottom.