Immobilized creatures are held in one place, but otherwise not restricted in what they can do. This effect cannot be resisted, as it was originally created for cut-scenes (where the module builder takes absolute control). Thus this effect is rarely encountered, but does sometimes get used in Bigby's grasping and crushing hands, as well as being one of the results of chaos shield. This effect is also applied within an entangle effect.
An immobilized creature is considered flatfooted, and its (fully modified) dexterity score is overridden to 3. This override is less damaging than it might appear, though, as the previous dexterity modifier is still used for all internal game calculations unless (or until) something else tries to affect the dexterity score. Furthermore, the AI of an immobolized non-player character is unable to perform actions other than a melee attack.
This effect is easier to dispel than most, as its creator's level is zero in the dispel check.