When something's jammed, it's either stuffed full or squeezed tightly into a small space. A sardine tin is jammed with little fishes, and a parking lot may be jammed with cars.
A jammed subway car is an ordinary rush hour sight in many big cities. The adjective jammed comes from the verb jam, "pack tightly." Its origin is mysterious, thought to be either related to champ, "chew noisily," or to be symbolic, possibly related to cram.