Passive Defenses are a type of Defensive Structure. Lacking any conventional weapon, they instead provide indirect assistance in protecting sectors from enemy attacks. Passive Defenses are often cheap, if not free to construct, and can usually be built by several Infantry units from each faction.
All factions can create obstacles - a type of Passive Defense that has no other benefit but to deny enemy movement through a passageway, or at least delay it. They either reroute enemies into pre-established killing zones, or prevent them from accessing an area altogether. Each type of obstacle denies movement to one type of unit: either Infantry or Vehicle. Many infantry units can set up this sort of Passive Defense to provide quick protection against enemy counter-attacks.
Mines are another common type of Passive Defense. They will simply explode when enemy units move over them. Most mines are devastatingly powerful, and are invisible to most enemy units. They are meant to punish the enemy if he tries to enter the protected area, and may even convince him to abandon the attempt and try to attack elsewhere.
Some Passive Defenses are meant only to provide cover for Infantry - whether by hiding behind or inside the structure. They will protect the infantry from Suppression, and usually reduce damage considerably. Enemy infantry who manage to reach such Passive Defenses can usually use them just as well as your own troops, so they can be double-edged swords if placed incorrectly.