A Force Field, often attached to a Combine Interface, prevents from passing through it any living thing that is not recognized. It is used prolifically by the Combine to control Citizen movements and prevent unauthorized access to areas that are out-of-bounds.
The Force Field consists of two pylon-shaped generators, each one on either side of the path it is intended to protect. Invisible from a distance (though it may be related to the game engine's worldmodel treatment), the Force Field preventing entry is generated between the two pylons.
When an unregistered individual attempts to go through it, it will emit a humming sound when physical contact is made. However, other than vehicles driven by non-recognized personnel, the system will not block inanimate objects.
Some of the Force Field are seen in-game being powered off standard human power supply sockets, Combine Power Generators, or from APC batteries. They are placed at regular intervals along railway tracks in City 17. Force Fields can also be found along the Coast, in the Citadel, in the Outlands, or any Combine-controlled area.
It is worth noting that while the player is merely stopped from moving into a Force Field, going closer will actually cause a discharge to other unauthorized persons, as seen in the Half-Life 2 chapter Entanglement where Alyx Vance attempts futilely to stop Judith Mossman making off with Eli Vance. In fact, being caught inside a Force Field when it is activated will result in instant death.
A smaller type of Force Field is used in Combine Bunkers to protect the soldiers inside while they man the Emplacement Guns.
The Depotteleport is closed by a Force Field barrier which gets open when a subject gets inside.