A choke or a choke point is a location on the map where open terrain narrows down into a smaller path, hence constricting unit movement and preventing surrounds. Spawn locations almost always have one to two choke points before opening up to the rest of the map. Most common among these are ramps, connecting higher and lower ground. Chokes can be a natural part of the map or artificially created by buildings or Force Fields.
A choke point makes it possible to create a wall off either with buildings, units or both and in this way blocking off your opponent.
Controlling a choke point gives you a strategic advantage since you can get a better concave around the other player's units. By positioning your units in firing range of the choke point you will have less idle units than the attacker in a battle moving through the choke. This can be used to defend the entry point to a base or to block a player from exiting the choke at your opponent's base, and this way a smaller army can hold off a much larger army that has to move through a choke.
Placing a unit or building with detection close to the choke point is a cost effective way of protecting your base from stealthy attacks.
Natural Choke Point
Any narrow path or enclosed area can be deemed a choke point, so long as it forces units to funnel together, rather than being able to travel abreast. Most maps have choke points at the entrance to the spawn locations as well as natural expansions. In addition, maps like Incineration Zone and Kulas Ravine have multiple choke points throughout the map, which players will try to position their forces near. These positional arrangements are subtle but can allow smaller armies to hold their own against larger armies. Ranged units have the advantage over lesser ranged at choke points.
Artificial Choke Point
An artificial choke point is a choke point that results from buildings, forcefields, units, invisible units, (etc.). A smaller area means that it is easier create a choke point.
Protoss can use buildings to further narrow an existing choke point, usually down to 1-2 squares wide. This is normally done to deny entry to high mobility light units like Zerglings and Hellions, and can be defended with fewer units. Zealots, set to "hold," can be incorporated into the wall to create a dynamic wall forcing melee units to have to focus fire the Zealot before they can pass. Photon Cannons can sometimes be a part of the wall or placed one square behind it utilizing the range advantage against melee and short-ranged units. In some cases (such as when the first Gateway is not complete), Protoss can use additional Pylons to create a complete wall-off, then cancel or destroy the Pylon later in order to move out.
In conjunction with Pylons, a cannon can also be used to create an offensive wall-off at the opponent's base.
Protoss are able to create an artificial choke when they have Sentries on the field. The Sentry can use its force field ability to block, narrow, and even create new choke points. In big army battles, this frequently is an essential part of Protoss tactics.
Terran has the most flexible way to gain tactical advantage from using buildings in choke points. With the ability to float off buildings and lower Supply Depots, a Terran player can fully control the size of the choke and can create a gate that selectively walls off a choke point. Since the range of a unit determines how many of its type can fire into a choke at the same time, the long ranged Siege Tanks are excellent units for controlling and defending chokes.
Zerg is the race with the least capabilities to create wall-off's or controlling chokes. Zerg players normally block and control choke points with units, preferably sturdy and cheap ones such as Roaches or Queens.
On maps like Lost Temple, Zerg players like to position Spine Crawlers and Evolution Chambers at the outside of the ramp to their Natural, forcing enemy units to swing around these structures and through a tight corridor blocked on the other side by a Hatchery.
Because Fungal Growth immobilizes enemy units, Zerg players can use the spell to defend choke points in a manner similar to Force Field. These "fungal fields" constrict or block movement in a choke point, deal damage, and allow for superior unit positioning.