There are a number of causes of lower extremity swelling. Probably the most common is venous insufficiency, in which the veins in the legs slowly lose their ability to return blood to the rest of the body. This is a very common and benign condition, and often this is due to slow degradation of the valves contained within the veins, making it easier for blood to pool in the legs when you are sitting or standing. If this is the case, then wearing support stockings during the day and keeping the legs elevated when you are sitting will typically resolve the problem.
Other less likely conditions to consider are early symptoms of heart failure, kidney disease and liver disease. Kidney disease can cause swelling through loss of protein in the urine, which allows fluid to seep out of the body's vessels and into the surrounding skin. Liver disease can cause lower extremity swelling (and classically, abdominal swelling) when the pressure in the veins leading to the liver builds up as a result of cirrhosis, where the liver is shrunken and scarred down. Lastly, if the ability of the heart to effectively pump blood is reduced, then fluid may build up behind the right side of the heart, leading to an accumulation of fluid in the legs and abdomen. You should talk about your swelling with your primary care doctor
to figure out how best to treat it in your individual case.