If anyone here has played Trials Fusion you will know what an area trigger is. Basically when you move into this area something will happen (i.e door open, explosion).
This will be very helpful for linear infection maps. After a certain amount of time the area trigger will spawn, door opens, and allow the payers to move forward.
When Bungie made forge improvements for Halo: Reach, they said they looked at some of the tricks the community used, and tried to make those be tools instead. Now hopefully 343 might do the same.