Sometimes a unit can by its presence alone affect the movement of unfriendly units in the vicinity, perhaps by requiring them to hide or to move carefully in order to pass by, or even to prevent entry altogether. This is called the "zone of control" or ZOC.
Exerting a ZOC requires no action, nor any particular capability on on the part of the unit exerting the ZOC. For instance, a toothless fort could still cause raiders to sneak by carefully (at least if they didn't know that it was toothless).