A choke point is an area of the map through which a lot of traffic flows, but the area itself is very restricted. For example, on Strike (Cod4 and MW2), the streets leading away from flag A (the highest flag, up the hill from B) are choke points. They offer little cover, but are the only exits from the A spawns. As a result, the other team can focus only on those points, because all enemy traffic will have to flow through them.
Now, that's a major choke point, but choke points can be even as simple as doorways. Doors are small, allowing only one or two people in at once, and restrict the aim and sight of those going through them. A person on the other side of a doorway can easily hold it, since anyone coming through is funneled and vulnerable.
Choke points prevent teams from rushing an enemy position (hence the "choke"), and are easy to control with concentrated fire. Choke points are most often used when defending an objective or camping.