This command enables or disables display of the track objects. The object editing commands become unavailable when objects are not displayed.
NFS3/HS tracks contain several kinds of objects:
Polygon objects: displayed in blue, are directly incorporated into the main track structure and are immovable.
Extra objects: displayed in green, are the only ones which allow the detection of a collision in the game, with the effect of either stopping the car (e.g. a tree) or moving the object (e.g. a road sign).
Global objects: displayed in teal, are usually animated objects (like the plane in Hometown); in NFSHS they are also used as objects cars can interact with (like cones or phone booths)
In order to edit global objects, you should convert them to extra objects first via the object's Properties dialog, then convert them back to global objects when you finish editing them.
Light sources: displayed as dark yellow Xs, they are basically the light flares displayed in the track, usually in the form of street lighting. In NFSHS they can also be used as special effects (smoke, steam, etc).
Sound sources: displayed as magenta speakers, they are used to deploy ambient sounds.