Deep vein thrombosis is a serious condition in which a clot occurs in the deep veins of the leg.
If this unhappy condition occurs it is usually following either leg injury or a period of enforced immobilisation. The reason why immobilisation can cause DVT is that the leg veins have no integral pump - the veins rely on the pumping action of the leg muscles to circulate the blood, with valves in the veins preventing the blood from sinking back down.
After injury or surgery the propensity of the blood to clot increases, and resting up diminishes the pumping action of the legs. Also a tourniquet may be used during leg surgery, stopping blood flow completely during this time. Thus surgery is a significant risk factor, and the big worry is that the clot may break free and shoot up to the lungs, causing ongoing breathlessness and even sudden post-operative death ('pulmonary embolus').